Astronomy

If the solar system is nearly flat, then why don't all the planets appear to lie on the same axis when viewed from earth?

If the solar system is nearly flat, then why don't all the planets appear to lie on the same axis when viewed from earth?


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For instance, consider the image below. The location of the planets seem to be so random relative to Earth. If the solar system was indeed flat, then I would expect the planets to lie on the same 'plane', and relative to us, it would look something like the second image. I mean, maybe Jupiter and Mars and Saturn could be on the same plane, but I fail to see how Venus would be.

What is wrong with my understanding here?


A plane looks like a curve when viewed on large angular scales on the sky (how could it be otherwise, think about a plane that cuts the north and south poles).

That plus the planets aren't in exactly the same plane.


The first image is from a July article by TV meteorologist Brent Watts. Approximating that view in Stellarium:

The yellow arc is the ecliptic, the plane of the Earth's orbit around the Sun. It looks curved due to the map projection. If the view is centered on it, the ecliptic looks straight:

Unlike the Sun, which appears on the ecliptic at all times, the planets can appear a few degrees north or south of it because their orbital inclinations are all slightly different. The more another planet's orbit is inclined relative to ours, and the closer it approaches us, the more its ecliptic latitude varies.


This Is Why We Aren't Expanding, Even If The Universe Is

If the Universe is expanding, we can understand why distant galaxies recede from us as they do. But . [+] then why aren't stars, planets and even atoms expanding, too?

C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47)

Take a look out at almost any galaxy in the Universe, and you'll find it's moving away from us. The farther away it is, the faster it appears to recede. As light travels through the Universe, it gets shifted to longer and redder wavelengths, as though the fabric of space itself is being stretched. At the largest distances, galaxies are being pushed away so rapidly by this expanding Universe that no signals we can possibly send will ever reach them, even at the speed of light.

But even though the fabric of space is expanding throughout the Universe — everywhere and in all directions — we aren't. Our atoms remain the same size. So do the planets, moons, and stars, as well as the distances separating them. Even the galaxies in our Local Group aren't expanding away from one another they're gravitating towards one another instead. Here's the key to understanding what is (and isn't) expanding in our expanding Universe.

The original conception of space, thanks to Newton, as fixed, absolute and unchanging. It was a . [+] stage where masses could exist and attract.

Amber Stuver, from her blog, Living Ligo

The first thing we have to understand is what our theory of gravity is, and how it differs from how you might think of it intuitively. Most of us think of space the way Newton did: as a fixed, unchanging set of coordinates that you could place your masses down onto. When Newton first conceived of the Universe, he pictured space as a grid. It was an absolute, fixed entity filled with masses that gravitationally attracted one another.

But when Einstein came along, he recognized that this imaginary grid wasn't fixed, wasn't absolute and wasn't at all like Newton had imagined. Instead, this grid was like a fabric, and the fabric itself was curved, distorted and forced to evolve over time by the presence of matter and energy. Moreover, the matter and energy within it determined how this spacetime fabric was curved.

The warping of spacetime, in the General Relativistic picture, by gravitational masses. Rather than . [+] a constant, unchanging grid, General Relativity admits a spacetime fabric that can both change over time and whose properties will appear different to observers with different motions and at different locations.

But if all you had within your spacetime was a bunch of masses, they would inevitably collapse to form a black hole, imploding the entire Universe. Einstein didn't like that idea, so he added a "fix" in the form of a cosmological constant. If there existed an extra term — representing an extra form of energy permeating empty space — it could repel all of these masses and hold the Universe static. It would prevent a gravitational collapse. By adding this extra feature, Einstein could make the Universe exist in a near-constant state for all eternity.

But not everyone was so wedded to the idea that the Universe needed to be static. One of the first solutions was by a physicist named Alexander Friedmann. He showed that if you didn't add this extra cosmological constant, and you had a Universe that was filled with anything energetic — matter, radiation, dust, fluid, etc. — there were two classes of solutions: one for a contracting Universe and one for an expanding Universe.

The 'raisin bread' model of the expanding Universe, where relative distances increase as the space . [+] (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by the time this light is received.

The mathematics tells you about the possible solutions, but you need to look to the physical Universe to find which one of these describes us. That came in the 1920s, thanks to the work of Edwin Hubble. Hubble was the first to discover that individual stars could be measured in other galaxies, determining their distance.

Nearly concurrent with this was the work of Vesto Slipher. Atoms work the same everywhere in the Universe: they absorb and emit light at certain, specific frequencies which depend on how their electrons are excited or de-excited. When he viewed these distant objects — which we now know to be other galaxies — their atomic signatures were shifted to longer wavelengths than could be explained.

When scientists combined these two observations, an incredible result popped out.

A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe . [+] that expanded faster in the past, but is still expanding today. This is a modern version of, extending thousands of times farther than, Hubble's original work. The various curves represent Universes made out of different constituent components.

Ned Wright, based on the latest data from Betoule et al. (2014)

There were only two ways to make sense of this. Either:

  1. all of relativity was wrong, we were at the center of the Universe, and everything was moving symmetrically away from us, or
  2. relativity was right, Friedmann was right, and the farther away a galaxy was from us, on average, the faster it appeared to recede from our perspective.

With one fell swoop, the expanding Universe went from being an idea to being the leading idea describing our Universe. The way the expansion works is a little counterintuitive. It's as though the fabric of space itself is getting stretched over time, and all the objects within that space are being dragged apart from one another.

The farther away an object is from another, the more "stretching" occurs, and so the faster they appear to recede from each other. If all you had was a Universe filled uniformly and evenly with matter, that matter would simply get less dense and would see everything expand away from everything else as time went on.

The cold fluctuations (shown in blue) in the CMB are not inherently colder, but rather represent . [+] regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies and clusters, while the underdense regions will be less likely to do so.

E.M. HUFF, THE SDSS-III TEAM AND THE SOUTH POLE TELESCOPE TEAM GRAPHIC BY ZOSIA ROSTOMIAN

But the Universe isn't perfectly even and uniform. It has overdense regions, like planets, stars, galaxies and clusters of galaxies. It has underdense regions, like great cosmic voids where there are virtually no massive objects present at all.

The reason for this is that there are other physical phenomena at play besides the Universe's expansion. On small scales, like the scales of living creatures and below, the electromagnetic and nuclear forces dominate. On larger scales, like those of planets, solar systems and galaxies, gravitational forces dominate. The big competition occurs on the largest scales of all — on the scale of the entire Universe — between the Universe's expansion and the gravitational attraction of all the matter and energy present within it.

On the largest scales, the Universe expands and galaxies recede from each other. But on smaller . [+] scales, gravitation overcomes the expansion, leading to the formation of stars, galaxies and clusters of galaxies.

NASA, ESA, and A. Feild (STScI)

On the largest scales of all, the expansion wins. The most distant galaxies are expanding away so quickly that no signals we send out, even at the speed of light, will ever reach them.

The superclusters of the Universe — these long, filamentary structures populated with galaxies and stretching for over a billion light years — are being stretched and pulled apart by the Universe's expansion. In the relatively short term, over the next few billion years, they will cease to exist. Even the Milky Way's nearest large galaxy cluster, the Virgo cluster, at just 50 million light years away, will never pull us into it. Despite a gravitational pull that's more than a thousand times as powerful as our own, the expansion of the Universe will drive all of this apart.

A large collection of many thousands of galaxies makes up our nearby neighborhood within 100,000,000 . [+] light years. The Virgo cluster itself will remain bound together, but the Milky Way will continue to expand away from it as time goes on.

Wikimedia Commons user Andrew Z. Colvin

But there are also smaller scales where the expansion has been overcome, at least locally. It's a lot easier to defeat the expansion of the Universe over smaller distance scales, as the gravitational force has more time to grow overdense regions on smaller scales than on larger ones.

Nearby, the Virgo cluster itself will remain gravitationally bound. The Milky Way and all the local group galaxies will stay bound together, eventually merging together under their own gravity. Earth will revolve around the Sun at the same orbital distance, Earth itself will remain the same size, and the atoms making up everything on it will not expand.

Why? Because the expansion of the Universe only has any effect where another force — whether gravitational, electromagnetic or nuclear — hasn't yet overcome it. If some force can successfully hold an object together, even the expanding Universe won't affect a change.

TRAPPIST-1 system compared to planets of the solar system and the moons of Jupiter. The orbits of . [+] everything shown here are unchanging with the expansion of the Universe, due to the binding force of gravity overcoming any effects of that expansion.

The reason for this is subtle, and is related to the fact that the expansion itself isn't a force, but rather a rate. Space is really still expanding on all scales, but the expansion only affects things cumulatively. There's a certain speed that space will expand at between any two points, but you have to compare that speed to the escape velocity between those two objects, which is a measure of how tightly or loosely they're bound together.

If there's a force binding those objects together that's greater than the background expansion speed, there will be no increase in the distance between them. If there's no increase in distance, there's no effective expansion. At every instant, it's more than counteracted, and so it never gets the additive effect that shows up between the unbound objects. As a result, stable, bound objects can survive unchanged for an eternity in the expanding Universe.

Whether bound by gravity, electromagnetism or any other force, the sizes of stable, held-together . [+] objects will not change even as the Universe expands. If you can overcome the cosmic expansion, you'll stay bound forever.

NASA, of Earth and Mars to scale

As long as the Universe has the properties we measure it to have, this will remain the case forever. Dark energy may exist and cause the distant galaxies to accelerate away from us, but the effect of the expansion across a fixed distance will never increase. Only in the case of a cosmic "Big Rip" — which the evidence points away from, not towards — will this conclusion change.

The fabric of space itself may still be expanding everywhere, but it doesn't have a measurable effect on every object. If some force binds you together strongly enough, the expanding Universe will have no effect on you. It's only on the largest scales of all, where all the binding forces between objects are too weak to defeat the speedy Hubble rate, that expansion occurs at all. As physicist Richard Price once put it, "Your waistline may be spreading, but you can't blame it on the expansion of the universe."


Why our solar system is flat

Of the imaginary coordinate lines that astronomers and navigators use in mapping the sky, perhaps the most important one is the ecliptic, the apparent path the sun appears to take through the sky as a result of the Earth's revolution around it.

Because of the Earth's yearly revolution around the sun, the sun appears to move in its annual journey through the heavens with the ecliptic as its path. Technically then, the ecliptic represents the extension or projection of the plane of the Earth's orbit out towards the sky.

But since the moon and planets also move in orbits, whose planes do not differ greatly from that of the Earth's orbit, these bodies, when visible in our sky, always stay relatively close to the ecliptic line. In other words, our solar system can be best defined as being somewhat flat, with the planets moving in very nearly the same plane.

It is for this reason that most sky charts plot the position of the ecliptic it is something of a warning to sky watchers that strange "stars" (planets) often appear near and along this path through our heavens, as well as the moon. Usually the moon and planets are not positioned exactly on the ecliptic (because they're not located exactly in the same orbital plane as Earth), but lie within several degrees of it and form a sort of narrow strip encompassing the entire sky which we call the Zodiac.

The ecliptic runs exactly along the middle of the Zodiac.

The 'Classic Twelve'
Twelve constellations through which the ecliptic passes form the Zodiac. The name is derived from the Greek, meaning "animal circle," and also is related to the word "zoo," coming from the fact that most of these constellations are named for animals, such as Leo, the Lion Taurus, the Bull and Cancer, the Crab, just to name a few.

These names which can be readily identified on sky charts are familiar to millions of horoscope users (who — ironically — would be hard pressed to find them in the actual sky!).

If we could see the stars in the daytime, we would see the sun slowly wander from one constellation of the Zodiac to the next, making one complete circle around the sky in one year.

Ancient astrologers were able to figure out where the sun was on the Zodiac by noting which was the last zodiacal constellation to rise ahead of the sun, or the first to set after it. Obviously, the sun had to be somewhere in between. As such, each month a specific constellation was conferred the title of "House of the sun," and in this manner each month-long period of the year was given its "sign of the Zodiac."

Some discrepancies
Interestingly, however, the "sign" which has been assigned for a given month in the horoscope that you'll find in your daily newspaper is not where the sun actually is for that particular month, but where it would have been several millennia ago!

This is due to the "wobble" of the Earth's axis (known as precession) yet today's astrologers, who believe that the sun, moon and planets mysteriously direct our lives, continue to adhere to star positions that for all intents and purposes are out of date by thousands of years!

Month in Space: January 2014

In addition, the ecliptic crosses through the constellation of Ophiuchus, the Serpent Holder. In fact, the sun spends more time traversing through Ophiuchus than nearby Scorpius! It officially resides in Scorpius for less than a week: from Nov. 23 through 29. It then moves into Ophiuchus on November 30 and remains within its boundaries for more than two weeks — until Dec. 17. And yet the Serpent Holder is not considered a member of the Zodiac and so must defer to Scorpius!

In addition, because the Moon and planets are often positioned either just to the north or south of the ecliptic, it allows them to sometimes appear within the boundaries of a number of other non-zodiacal star patterns.In fact, as pointed out by the well-known astronomical calculator, Jean Meeus, along with Ophiuchus, there are nine other constellations that occasionally can be visited by the Moon and planets: Auriga, the Charioteer Cetus, the Whale Corvus, the Crow Crater, the Cup Hydra, the Water Snake Orion, the Hunter Pegasus, the Flying Horse Scutum, the Shield and Sextans, the Sextant.

So in truth, there really aren't twelve zodiacal constellations, but twenty-two!

Origin of 'Ecliptic'
Although the moon's orbit is inclined 5.5 degrees to the Earth's orbital plane, periodically there will come times when it crosses over the ecliptic.

Should this happen when the moon is at new phase, it will end up crossing in front of the sun causing a solar eclipse. If the moon crosses over the ecliptic when the moon is at full phase, it will pass into the shadow of the Earth resulting in a lunar eclipse. Usually when the new moon is in the vicinity of the sun it appears to pass above or below it and no eclipse occurs. Similarly, the full moon usually misses the Earth's shadow by sweeping above or below it.

Only when all three bodies (sun, Earth and moon) are on a straight line occupying the plane of the ecliptic can an eclipse occur.


Movement Of The Earth

Earth movement : the earth moves in space in two distinct ways, these two distinct ways are

1] The Earth rotates on its own axis from west to east, that is, in a clockwise direction once in every 24 hours causing day and night.top↑

2] The Earth also revolves round the sun in an orbit once in every 365¼ days causing the seasons and the year.

Rotation Of The Earth

1] The earth rotates or moves on its axis from west to east.

2] The rotating earth is inclined at an angle of 23 1/3° along its axis.

3] It takes the earth 24 hours, that is, a day to complete one rotation.
Meaning, the earth rotates through 360° in every 24 hours.

4] The rotation of the earth causes day an night.top↑

5] There are 24 hours in a day, hence, there are 12 hours of day light and 12 hours of darkness.

6] The earth rotates through 15° in 1 hour and rotate through 1° in 4 minutes.

The sun does not move but it appears as if the sun is moving, sometimes the sun disappears completely which is called the apparent movement of the sun

Effects Of Rotation Of The Earth

1] Day and night : as the earth rotates on its axis, only one part of the earth faces the sun at a time and receives day light from the sun while the other part of the earth backing the sun rays will experience total darkness.top↑

2] Time difference from place to place : The earth rotates through 360° in every 24 hours and every 1 hour the earth rotates through 15°, therefore, generally there is a time difference of 1 hour for every 15° of longitude when some people are experiencing sunrise, other are experiencing sunset, noon and other midnight.

3] Deflation of rain and ocean current : due to the rotation of the earth, freely moving bodies on the surface of the earth are deflected from their original path to the right in the northern hemisphere or to the left in the southern hemisphere.top↑

4] Daily rising and falling of the tides : the earth rotates explain the gravitational pull exerted by the sun and the moon, causing daily rising and falling of the tides.

The fifth one is , dawn and twilight, I believe I have explain what a dawn and what a twilight is? But if you are new to this session, here is the link to the session The Solar System, there you will see everything, till next class guys.

Don’t forget to hit the share button.top↑

Feel free to ask our tutors any question on The Shape Of The Earth And Its Proofs via the comment box or ask question page and I will be happy to answer your question and take the session again if necessary, your comment notifications is a motivation to us, so don’t forget to write something on The Shape Of The Earth And Its Proofs.


The Institute for Creation Research

Although Venus has been called Earth&rsquos sister due to the similar size of these two worlds, the planet that appears most earthlike at its surface is undoubtedly Mars. A solid, rocky world, Mars is just over half the size of Earth in diameter. It appears as a vivid red star in our nighttime sky, giving rise to its nickname, the Red Planet. This is no illusion. The surface of Mars is composed of oxidized compounds of iron&mdashessentially rust. This amazing planet has properties that both challenge secular ideas and confirm biblical creation. Its similarities to Earth make it a tempting target for enthusiasts of space colonization however, the stark differences between the two planets should dampen such enthusiasm.

A Day on the Surface of Mars

Geologically, Mars has features strikingly comparable to those on Earth. With mountains, valleys, canyons, volcanoes, and polar ice caps, Mars even has some weather similar to Earth&rsquos, including seasons, clouds, fog, wind, dust storms, dust devils, and occasional frost. Although liquid water is not found in any abundance on Mars, scientists have discovered substantial quantities of water-ice near the poles and water vapor in the Martian atmosphere. Even the axial tilt and rotational period of Mars is much the same as that of Earth.

Mars takes 24 hours and 37 minutes to rotate once on its axis&mdashalmost identical to Earth. 1 Future visitors to the planet might find this slightly longer day enjoyable. They could sleep a half-hour longer compared to their friends on Earth, and it would never &ldquocatch up&rdquo with them. The sun would appear slightly smaller than it does on Earth and would shine at only half the brightness since Mars orbits farther from the sun. As a result of this larger orbit, the Martian year equals 1.9 Earth years.

With a thin atmosphere composed mainly of carbon dioxide, Mars does not provide a breathable environment for humans. In order to survive, future astronauts would have to wear spacesuits with built-in oxygen supplies when walking on the Martian surface. The spacesuits would also need to provide the atmospheric pressure and temperature necessary for human life. Because the force of gravity is only 38 percent of that on Earth, walking on Mars would take some practice. By comparison, the astronauts on the moon experienced a force of gravity only 17 percent of what is normal on Earth.

In the daytime, visitors on Mars would experience a bright sky, though not as bright as Earth&rsquos. The color of the Martian sky is interesting, complex, and often blue for exactly the same reason that Earth&rsquos sky is blue: The molecules in the atmosphere scatter shorter wavelengths (blue) more readily than longer wavelengths (red). But the Martian sky is a deeper, darker blue&mdashpartly because there is less sunlight than on Earth and also because the atmosphere is much less substantial. When wind kicks up fine dust from the planet&rsquos red surface, the Martian sky can also appear to be orange.

Martian Seasons

Due to sharing a similar axial tilt to Earth at 25.2 degrees, Mars also experiences four seasons. Observers on the planet&rsquos surface would see the sun high in the sky in summer and low in the sky in winter and would experience the same amount of sun exposure as they would at comparable latitudes on Earth during these seasons. Seasons on Earth result from axial tilt, not the changing distance to the sun caused by Earth&rsquos slightly elliptical orbit. This is also true for Mars however, the orbit of Mars is significantly more elliptical than Earth&rsquos, which causes its distance from the sun to change, affecting the severity of its seasons. So, even though, like Earth, Mars is closer to the sun during its northern hemisphere winter and farther away during its northern hemisphere summer, the effects are different. Its greater distance to the sun partially compensates for the increased duration and direct angle of sunlight experienced in northern hemisphere summers. And while Earth&rsquos elliptical orbit barely affects the extremity of its seasons, the elliptical orbit of Mars causes seasons to be less extreme in its own northern hemisphere than in its southern hemisphere.

In addition, Mars has polar ice caps that are visible from Earth using a small telescope. 2 These ice caps grow during the winter in their respective hemispheres and shrink during the summer&mdashjust like the ice caps on Earth. But Earth&rsquos ice caps are water-ice, and Mars&rsquo ice caps are mostly water-ice layered underneath several feet of frozen carbon dioxide (dry ice).

Martian Topography

Mars is flat&mdashvery flat. Most of its surface resembles the deserts we have on Earth, with rocks as far as the eye can see and very little relief. Though there are hills and even enormous mountains, they have gentle slopes that make them seem less magnificent than peaks on Earth. For example, Olympus Mons is a massive (extinct) Martian volcano and is actually the largest volcano known to exist&mdashnearly three times as tall as Mt. Everest. Yet, even though its base would cover the combined states of Ohio, Indiana, and Kentucky, a mild gradient makes Olympus Mons seem far less impressive than the rugged slopes of Everest. Several other immense volcanoes exist on Mars, dwarfing their terrestrial counterparts. Most astronomers believe that all of these volcanoes are extinct and that Mars currently has essentially no geologic activity.

One of Mars&rsquo most spectacular features is a canyon called Valles Marineris that is long enough to reach from one end of the United States to the other and is over 120 miles wide and about four miles deep. 3 For comparison, this is ten times longer, nearly seven times wider, and four times deeper than the Grand Canyon. Valles Marineris is thought to be a tectonic fissure&mdasha place where the surface cracked open. 4

Scientists have been intrigued to learn that the surface of Mars has dry river beds and deltas. Though there is essentially no liquid water on the planet today, evidence clearly suggests that Mars once had surface water. Such evidence is especially perplexing in light of the planet&rsquos thin atmosphere. Water can only exist as a liquid between certain temperatures and under sufficient atmospheric pressures, and the atmosphere of Mars is far too thin to allow water to be liquid for any length of time at any temperature. Heating an ice cube on Mars would cause it to sublime, not melt. That is, the ice would go directly to vapor, bypassing the liquid state entirely. Frozen carbon dioxide behaves in the same way under Earth&rsquos atmosphere.

So, was the atmosphere of Mars different in the past? Or was the water released catastrophically, boiling away almost immediately? Could volcanic eruptions increase the atmospheric pressure locally to the point where liquid water could exist temporarily? These are mysteries that remain unsolved. It is noteworthy that secularists are willing to believe in catastrophic, planet-scale flooding on Mars&mdasha planet that cannot support liquid water. Yet, they simultaneously deny the Genesis Flood on Earth&mdasha planet that is 71 percent covered with water.

Martian Moons

The two moons of Mars are quite tiny compared to Earth&rsquos moon. Phobos is the larger of the two and only about 10 miles in diameter. Since Phobos has so little mass, its gravity is minuscule. In fact, you could pick up a baseball and toss it into orbit around Phobos. And, if you threw it just right, you could turn around and catch it as it completed a loop! 5 Deimos is the other Martian moon and has a diameter of only eight miles. 6 More like two large boulders orbiting Mars, neither Phobos nor Deimos is spherical. This is common with small moons and asteroids since their gravity is insufficient to overcome the chemical bonds that prevent these bodies from collapsing into a spherical shape.

Phobos and Deimos have very circular orbits&mdashboth quite near to the Martian surface. Phobos orbits at an unbelievably close distance of only 3,700 miles above the surface&mdashcloser than any moon to its planet. To stand on this little world of Phobos and look up at an enormous Mars would be a truly spectacular sight. Its proximity to Mars&mdashcombined with Mars&rsquo gravity&mdashmeans that Phobos orbits very quickly. In only 7 hours and 39 minutes, this little moon can complete one orbit. A greater distance away from the surface, Deimos takes just over 30 hours to complete one orbit. Since Phobos orbits faster than Mars rotates, an observer on the Martian surface would actually see Phobos rising in the west and Deimos rising in the east (albeit very slowly), despite the fact that both moons orbit Mars in the same direction!

When taken from a secular perspective, the origin of these moons is perplexing. Were they once asteroids that have since been captured by the gravity of Mars, as many astronomers believe? This is possible but involves an improbable chain of events. Moreover, captured asteroids are expected to have exaggerated, elliptical orbits, but Mars&rsquo moons orbit in nearly perfect circles. As with so many aspects of the universe, the creative diversity of the Lord seems the best explanation for this puzzle. While posing a challenge for natural processes, the creation of unique moons in well-designed orbits is no problem for God.

Martian Opposition

Outer planets (those beyond Earth&rsquos orbit) are best viewed through a telescope when Earth passes between them and the sun. This is because the outer planet is about as close to Earth as it can be, is fully illuminated by sunlight, and is high in our sky around midnight when the sky is darkest. During such a configuration, the outer planet is said to be in &ldquoopposition&rdquo because it is opposite the sun. But most outer planets still appear large and bright even when they are not in opposition, which happens about once per Earth year. Mars is the exception to both of these generalities.

Because it is so small, the planet only looks bright (and large in a telescope) for a month or so around opposition. And unfortunately, because its orbital period is nearly twice as long as Earth&rsquos, Mars&rsquo opposition only happens an average of once every 2.1 years. So don&rsquot miss it. 7 During opposition, Mars comes very close to Earth, which is why it looks so good, appearing in a telescope seven times larger and 50 times brighter than it does when on the far side of the sun. By contrast, Jupiter always looks about the same size and brightness, whether in or out of opposition, because it is a large planet and is only slightly closer to Earth at opposition than at other times.

Not all of Mars&rsquo oppositions are equal. Since its orbit is quite elliptical, some oppositions bring the planet much closer to Earth than others. Mars can appear nearly twice as large during favorable oppositions as in unfavorable ones. In fact, on August 27, 2003, Mars and Earth came as close together as they ever have&mdash34.6 million miles&mdashabout as close as is possible for these two worlds. This led to some wonderful telescopic views of Mars. 8

Additionally, it is only when Mars is near opposition that the moons Phobos and Deimos are visible under good, dark conditions with a moderately sized backyard telescope. Even then, it can be a challenge. The problem is not so much that these moons are faint&mdashbackyard telescopes can resolve stars significantly fainter&mdashbut that they are so close to Mars, which is 200,000 times brighter and covers them under its glare. The best way to see Phobos and Deimos is to move the telescope so that the moons are within the field of view and Mars is just beyond it.

Mars and Earth possess great similarities but also vast differences. This is yet one more mark of the creativity of the Trinitarian God of Scripture. God Himself (Father, Son, and Holy Spirit) embodies a multitude of characteristics&mdashdiverse and yet unified. In the same way, the planets, while not one-and-the-same, have unique variations representing the all-encompassing, endless ingenuity that the Creator exemplifies in all His forms. Indeed, the evidence of Him is clearly seen by what He has made&mdash&ldquoeven His eternal power and Godhead&rdquo (Romans 1:20).


Disclaimer: The following material is being kept online for archival purposes.

Please note!

    Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection-- of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.

325. Tapping Atmospheric Electricity

I would like your views on a proposed new Atmospheric Power Generation (APG) technology. A device containing a large numbers of radioactively treated needles in an electron emitting device is elevated into the atmosphere by an airship, and is connected to a ground-based power converter through a conductive tether. Electrons are drawn from the ground, flow through the power converter and tether, and are emitted into the atmosphere by the needles. The voltage field drawing electrons from the needles is the atmospheric voltage gradient, and the current flow is affected by air conductivity at the applicable altitudes.

It has been proposed that the maximum amount of power we would be able to extract from the atmosphere globally will be limited by the total power normally residing in positive ions within the global atmospheric circuit. This number is about (250KV)*(1,000A) = 250MW.

We would be very appreciative if you would share any of your thoughts on any of the questions and possibilities above.

Reply

Please tell your partnership that this idea is not likely to be useful.

What you have in mind, as I understand it, is an electrical circuit which taps the atmospheric potential at an airship moored at some altitude, sends a current through the mooring cable to the ground (where some electric power can be utilized), and closes the circuit back through the atmosphere, to the region around the airship where, presumably, more electric charge is waiting to be neutralized.

Just apart from the electric aspects, let me state that an airship moored at even 1 kilometer is a technological challenge. The cable and its insulation will be heavy, especially since it has to both moor the airship and carry its own weight, forces which stretch it in opposite directions. The airship--if filled with helium, will be expensive, if with hydrogen, it must be very gas tight, and in any case, airships at that altitude are very vulnerable to storms (think of lightning, too).

But beyond that, the atmospheric electric field contains rather little energy. It is maintained, if I understand, by the global distribution of thunderstorm clouds. Suppose you remove some of it, how soon will it be replenished? You close your circuit through the atmosphere at 1 km below, through the weak conductivity of air (mostly caused by cosmic rays). However, the thunderstorms replenishing the potential may be hundreds if not thousands of kilometers distant, through the same poorly conducting air!

326. Global disaster in 2012?

Reply

I have received a substantial number of questions like yours--and this is only 2007! Rest assured, no disaster is foreseen on that date. Rather than answer you specifically, let me direct you to my questions-and-answers section, e.g.

and look up there questions #306, #291b, and #264 (and their replies), also, if you wish, #302.

. . . . . Continuing the question

Thanks. It is just that my son has done a lot of searching on this topic and stuff about a black hole, a pole shift, gamma ray bursts, asteroids and many coincidences have come up. Everything leads to 12/21/2012, including a web program predicting the end of the world on that day and the date showing up in I Ching.

Are all these predictions just pseudoscience and authors bending the truth or is there actual scientific evidence about what's going to happen in 2012? Also, is the Earth's magnetic field really weakening and the earth's rotation really slowing?

Reply

As far as I can see, there is no physics or astronomy involved, just a lot of astrology. It is not a lie, it is just superstition.

The Earth's main north-south field is weakening, as it has been for centuries, by about 5% per century, maybe now speeded up to 7%. Scientists do not regard it as unusually fast. The field may reverse polarity in 1500 years or so--or else, its slow change may rise again. Records in ancient lavas tell that both of these have happened.

The Earth's rotation is slowing down because of the attraction of the Moon and Sun, raising tides on Earth. Now and then the total accumulated slowing down adds up to a second, and master clocks are reset accordingly. This happens at intervals of years (see Wikipedia. after asking Google about "leap seconds") and is also rather slow.

327. What's the difference between speed and velocity?

Reply

    'When I use a word,' Humpty Dumpty said, in a rather scornful tone,' it means just what I choose it to mean, neither more nor less.'
    'The question is,' said Alice, 'whether you can make words mean so many different things.'
    'The question is,' said Humpty Dumpty, 'which is to be master --that's all.'

328. Effect of Gravity on Electromagnetic Waves

Reply

An EM wave such as light is distorted very slightly by gravity. It takes a very strong gravitational field to produce a distortion sufficiently strong to be observable. For more, see

329. Why is North the reference, not South?

Reply

Our maps originated around the Mediterranean, well north of the equator. In those countries, the north star or a spot near it in the sky (see http://www.phy6.org/stargaze/Spolaris.htm ) is the pivot around which the stars seem to turn at night, and the sun in the day. North is therefore a direction easy to establish at night. It is also the direction of the shadow of a sundial at noon, when the shadow is shortest.

Naturally, it was taken as reference direction. Interestingly, though, when the ancient Chinese discovered the magnetic compass, they viewed it as pointing south. Since the compass needle points (very nearly) north-south, deciding which of these directions is to serve as reference can be one's own choice.

330. The lowest 700 km of our Atmosphere

I have been looking through your website with much interest.

I am an artist based in Notttingham England and for the last year I have been conducting a long-distance collaboration with an artist in Toronto, Canada. Our most recent project is a performance walk through our cities inspired by the idea of climbing enough stairs to see each other over the horizon.

We have been helped with the calculations and are aiming to climb the 699km calculated, using stairs. This of course will take us a long time and so this is an ongoing project that will happen in stages. This weekend we start another stage of our journey. We are taking an audience on our walks this time, and I would like to tell them more about what it would be like at the height we are aiming for. Can you give me any pointers on where I could find information like this, or can you help me

Reply

The atmosphere at 699 kilometers is at most a trace of what you and I breathe (and is mostly hydrogen). Where we live, of course, the air is compressed by the weight of all the layers above it, and as you rise, that weight decreases (some is now below you), and the pressure decreases in proportion.

Observations suggest that about half the atmosphere is contained in the lowest 5 kilometers, so at that altitude, air density is down to one half. And so with the next 5 kilometers, and the 5 km after that, and so on (temperature affects that distance too but only moderately). At 10 kilometers you therefore have 1/4 of the pressure and density. At that altitude or a little higher is where most weather processes end. Weather is essentially the process by which the atmosphere removes heat from the ground (where sunlight has put it) and transports is upwards--winds, clouds, rain etc. all play a role, and of course air gets steadily colder, because heat only flows from hot to cold.

At 10-15 kilometers (with one small exception) that process ends, the atmosphere becomes transparent to heat radiation (infra-red) which then flows out into space. It also gets very dry, so when you fly on a jet, you see most clouds below you. The exception is the ozone layer at 25-40 kilometers, which absorbs ultra-violet light and causes some local heating.

If you halve the density every 5 kilometers, at 100 kilometers only about a millionth of the air density the ground level is left, and soon after that collisions between molecules become infrequent. Molecules still collide, but they do not mix as well. Near the ground air is about 78% nitrogen, 21% oxygen and 1% argon. Above 100 kilometers or so, heavier particles rise less. Atomic oxygen (split up by sunlight) is lighter than oxygen molecules and rises fairly high, but helium and hydrogen, being even lighter, outlast all others, even though their part in the atmosphere is very small. Earth is thus surrounded by a big "geocorona" of hydrogen, which astronauts have photographed in the ultraviolet color which hydrogen scatters and in which it glows--see

But that is just the "neutral" atmosphere. Sunlight also separates electrons from atoms to form positive "ions", surrounded by free electrons. Below 100 kilometers these collide and recombine--new ones are created in the daytime, but at night the ion layer ("ionosphere") recombines and disappears. Above about 140 kilometers, the collisions of ions are less frequent, which causes them to be bound mostly to magnetic field lines: electric currents flow well along such lines, but have difficulty jumping from one line to another, that is, flowing horizontally.

At 200 kilometres and higher, the ionosphere does not recombine, but its density goes down with distance, since most ions are still held by gravity. Around that level collisions become rare and molecules rise and fall like tossed stones. There are up to a million ions and electrons per cubic centimeter at that altitude (the numbers must match), but only about 100,000 at 600 kilometers.

In addition, some electrons and ions acquire much higher energy by processes having to do with the Sun and the solar wind. They are much too fast to be held by gravity, but magnetic forces which originate inside Earth can trap them. Their number is not large--but they can be interesting, since they carry electric currents which modify the magnetic field, and at the highest energies they constitute the Earth's radiation belt. At places the edge of this belt may come down to 699 kilometers or so, but between Nottingham and Toronto you are likely to be just below the radiation belt, and the "radiation" will not reach you.

That is a very quick description of the region you plan to "cover." And by the way, it would be hard to climb as many stairs as you plan without repeating some, even if you do much of the climbing indoors. My parents in New York lived on the 10th floor of a building with 36 floors, and sometimes I climbed up 35 floors and went down by elevator, just for exercise. It was rather boring. And you would need a thousand such climbs just to gain 100 kilometers. Good luck with your project!

331. Doomsday 2012?

I'm sure you will probably be laughing by the time you get to the end of this e-mail, but the answer to the question is important to me.

My husband was watching some sort of television show that was promoting the idea that the world might end in 2012. The evidence provided was that this is the year the Mayan calendar ends and that planets will align with a black hole, thus magnetic pole reversal will occur. Hence, life here on earth will end as we know it - catastrophic flooding, radiation, etc.

I don't know much about the earth's magnetism, although I have been reading about it since I stumbled upon your website after I finally went to NASA's website. Most of the information on the internet regarding 2012 presents doomsday predictions relating to the pole reversal.

My question: Is there any evidence that a magnetic pole reversal will occur in 2011 or 2012? Is this something that has been newly discovered?

From what I understand from your website, a pole reversal will probably not occur for at least 1200 or so years and this is only if current trends hold true. In addition, it seems that we always have some magnetism during a pole reversal, it is only that the magnetism is weaker.

Reply

It constantly amazes me how superstitious some people are. How should the Maya know about magnetic reversals, if they had no way of being aware of magnetism--no iron, even, they were essentially still in the stone age! How should they have known about black holes, a concept requiring information about stars and gravity which even in our civilization (with its telescopes) are less than a century old? Yes, they had a very accurate calendar, but so did the priests of ancient Babylon.

Incidentally, you seem to have understood well what you read on my site about reversals. They are not expected to not change anything important for life on Earth, and as noted, at the present rate one will not happen sooner than in 1000 years. Even that is uncertain: the geomagnetic record of old lavas suggest that the "main field" of Earth fluctuates constantly, and in the past "excursions" have taken place when the field seemed to approach a reversal, but then changed to an opposite trend.

332. Where does a Flying Bird get its Support?

My class and I have a physics question. Could you please give a scientific answer?

A large bird cage is attached to a digital scale, and the mass on the scale reads 20 kg. A bird is sitting on a perch in the cage.

The bird then gets off the perch and flies around the cage without touching anything. Will the mass reading on the digital scale change?

Reply

Ask yourself: when the bird flies around, what keeps it aloft? By Newton's 3rd law, if it is held up by an upward force of 20 kg (or an appropriate number of Newtons, pick your favorite units), an opposite and equal downward force must be applied SOMEWHERE. My guess is that the wings of the bird cause a downward motion of air, with an equal force.

(Another example a propeller drags an airplane forward with force F. We know that an equal and opposite force must act on the air, and it does--the well-known "prop wash" blowing air backwards. And if a helicopter hovers above water, you can see the force of that wind on the surface of the water, too.)

So the bird must constantly push air downwards, with a force of 20 Kg. What sort of cage is it in, anyway? If the cage has a solid bottom, I suspect that force is transmitted to it and from it to the scale. There should be little or no change in the registered weight.

If however the bottom of the cage consists of loose wires, air blown down through it passes between the wires to the air below the cage and ultimately, to the air in the rest of the room and to the floor. The registered weight should then be much less.

Confronted with such a question, always ask yourself--what IS going on?

333. Why does Sun seem to move?

Reply

I do not know what books your teacher recommended, but YOU have my web sites, and all answers are there. Just read them! (And tell your teacher. There also exist lesson plans)

Briefly, there exist two motions of the Sun across the sky ("apparent" motions, because we are the ones who really move). The daily motion --rising in the east, setting in the west--happens because the Earth rotates around its axis. The Moon, and the stars seen at night, all have similar motions. If the Sun were above the Earth's equator, it would trace the same path day after day.

In addition, over the year the Sun's position among the stars traces a big circuit, around a circle which makes an angle 23 1/2 degrees with the equator. That is why we have seasons . In summer the Sun is in the north part of the sky, days are longer and we have summer (in the USA and Europe). In winter it is south of the equator--long nights, short days, cold weather.

The line among the stars which it follows is known as the ecliptic . We cannot see where the Sun is among the stars, but we can guess its position by comparing it to the stars seen at night, which change over the year (on any day, the Sun is among those stars we cannot see ). That motion is caused by the orbit of the Earth around the Sun.

334. Why don't waves disturb each other?

I am YS from Malaysia, a physics degree student. I came across your website at phy6.org while I was searching for an answer to my question.

I've been learning about waves and superposition and I am very familiar with the effects when two waves meet. However, just recently, I realized that I don't know what happens after the two waves meet (waves traveling in opposite direction, heading towards each other).

I am surprised to learn that after two waves (traveling towards one another) meet, they will continue their paths as if nothing happened after the superposition incident. Do you know why does this happen?

I manage to find many websites that explain the events before and during the superposition. But I haven't manage to find any website with a detailed explanation (with theoretical explanation, mathematical calculation, etc) to the events after the superposition.

I do hope you can help me out or at least direct me to a website. And I really appreciate all the effort and knowledge you have shared in your website.

Thank you and have a wonderful day. Warmest Regards from Malaysia,

Reply

Superposition is a mathematical property derived from linearity. If you have a linear equation

and you have some (x,y,z) that solves it (for instance, [1, 2, 𔃀]), , and you also have a completely different solution (X,Y,Z) (for instance, [𔃀, 4, 𔃀]) which also solves it, the sum of the solutions, and indeed any linear combination (3 times the first minus 5 times the second, say) is also a solution.

If your equation has powers or multiplication of unknown numbers, for instance

that property does not exist, making the equation "nonlinear."

Waves satisfy a linear " wave equation ." That equation involves calculus rather than being algebraic like the ones above, but many similarities still hold. In particular, if you add two waves in any fixed proportion, the result also satisfies the equation (you can even subtract), and creates the sum of the effects. You sit in a room and the radio is playing while you talk to a friend, and the sounds propagate independently. You shout to a friend across a ballfield while the friend shouts at you--you hear your own voice first and then the friend's, and with the friend it is in opposite order. The two sounds cross each other unchanged.

Not all phenomena are linear. Waves on the surface of water are. So are electromagnetic waves--which is why many radio and TV stations can share the same space. Shocks are not linear. If you want to examine the mathematics, start with a wave moving in one dimension, which has relatively simple solutions.

335. Does the Moon's motion Change?

Reply

The Moon's motion is not far from the ecliptic, which rises high in the sky in summer and is lowest in winter: its motion therefore changes with the seasons somewhat like the Sun's.

The ellipticity of the Moon's orbit cause some changes in its apparent size, but not by much--I do not think people notice.

And the appearance of the Moon changes with position. We see a "Man in the Moon", but in Argentina the appearance is different--not because the position of the Moon differs, but because the "up" direction does.

336. Big Dipper and Weather

Hello from Newfoundland, Canada

I was just wondering if there was any truth about the handle of the Big Dipper . they say that when the handle is up it means nice weather and when the handle is down it means really bad weather. Please email me and let me know what you find out. Thank you very much.

Reply

I have not heard that story before and cannot think of a reason why it would hold true. The Big Dipper of course goes around the pole of the sky every 24 hours--I think right now its tail points east-west, and then towards morning it points up, while a few hours ago it slanted down.

You might think of that tail as the pointer of a 24 hour clock. If you know whether one position of that clock should give different weather than another (in particular, noting that any kind of weather usually persists longer than 12 hours), you might have an argument.

337. What IS the Ecliptic?

Reply

It is neither. It is the flat plane of the orbit of the Earth around the Sun.

Draw in your mind a line from Earth to the Sun. As the Earth orbits the Sun (in one year), Earth and Sun are always at opposite ends of that line. So seen from Earth, the Sun moves in the plane of the ecliptic, too, and its path among the stars is also in the ecliptic.

The line traced by that path among the stars is ALSO called the ecliptic, and is so labeled on star charts. The 12 constellations along this path are the famous "zodiac" and each corresponds to a month.

But there is more! The ENTIRE SOLAR SYSTEM is flat, and the planes of the orbits of all major planets (and of our Moon, too) are all near the ecliptic, usually just a few degrees off. So to the casual eye, the Sun, Moon and planets ALL seem to move (more or less) in the ecliptic. If you step out on a clear evening, the Sun has just set at some spot on the horizon, the Moon is some distance away, and there are some bright stars right on the line between the two--these are probably planets, because all these objects move in (almost) the same flat plane, and we on Earth view this plane edge-on.

338. Precession, Greenhouse and more.

I'm really enjoying your web page, you seem to have a lot of info at hand. Maybe you will find some of my questions of interest, and stop them from waking me up in the middle of the night.

I tried to explain the precession of the earth's axis to some friends of mine. Which of the following is correct?

A) In 13000 years summer in the northern hemisphere will be in December instead of June, as supported by the picture on your web page http://www.phy6.org/stargaze/Sprecess.htm

B) The calendar is adjusted so summer will always be in June, although the background of space in summer will be the same in 13000 years as it is now in winter (as supported by your answer to http://www.phy6.org/stargaze/StarFAQ11.htm#q174)

1) The southern hemisphere is slightly warmer (can't remember how much) than the northern because when it's summer in the southern hemisphere the earth is closer to the sun because of the elliptical orbit. In 13000 years, the north should be slightly warmer than the south - but again, the question is how much?

2) How much energy does the sun deposit on the earth in a year, and how much energy do humans dump on the earth in a year (through all power generation sources and fuel burning activities). What percentage of heat on the planet comes from the sun and what percentage is from humans?

3) I'm also fascinated by the whole global warming / clean energy arguments - my view is that if you really believe CO 2 is the root of all evil, you should simply replace all power plants with nuclear plants, and then build more nuclear plants to drive electric cars, heating, etc to replace burned fuels. Somehow I don't believe that will cool off the planet, but it may help if I knew the answer to my question # 2 above.

Here's another one - If I grow sugar cane (which pulls carbon out of the air), harvest it, make ethanol, burn that for energy in cars (and in the sugar cane / ethanol processing plant), do I have a net zero carbon contribution to the environment even though I am burning fuels? I probably end up putting a lot more water vapor into the air though.

How about this - solar cells are about 10% efficient (varying widely depending on type). Does that mean they absorb 90% of the energy that hits them as heat which gets radiated off in all directions? If I paint my roof highly reflective white and keep as much of the heat from the sun as possible away from it, and my neighbor covers his roof with solar cells and converts 10% of the energy to electricity and then runs his air conditioner with that to cool the house (which has 90% of the energy radiating all around it), who ends up with the net lower energy footprint? Are solar cells really environmentally friendly if they create a big black footprint that converts 90% of the energy that hits them to heat near the ground that gets radiated vs a more reflective surface or maybe an agricultural area growing sugar cane for example?

Reply

It is sure easier to think up new questions than answering them! But I will try

  1. Yes, in 13000 years, we will be closest to the Sun in northern midsummer. I do not know however how much effect this will have. It may make the northern climate zones slide equatorward two degrees or so--or this may be overtaken by other effects.
  2. This can be estimated (have your calculator handy!). The radius of the Earth is 6.371 10 6 meter, so the cross section presented to the Sun is about 1.27 10 14 meters squared. Each meter squared perpendicular to the sun's direction gets about 1370 watt (the "solar constant," fluctuating slightly because the orbit is elliptical), so we have 1.74 10 17 watt hitting the Earth.

Well. you still come ahead in one sense. The white paint reflects visible light, to which the atmosphere is transparent, while his hot roof radiates infra-red, which has to run the gauntlet of absorption and re-emission, an effect which the greenhouse effect enhances. However, I doubt it whether you can keep your house passively as cool as an air conditioner would do (plus, you can always add insulation--and an air conditioner dries up the air, too).

339. Latest Sunrise, Earliest Sunset

After reading the "Stargazers" sites on Kepler and ellipses, etc., I was wondering about one thing that I haven't been able answer.

I understand why, as you point out, the winter season in the north is shorter than the summer season (equinox to equinox) (Kepler's second), and that the solstices and equinoxes do occur on approximately the same dates every year.

But then I found out that the day of the year with the earliest sunset (in the afternoon/evening) occurs in early December, and the latest sunrise occurs in early January. The corresponding thing happens around the summer solstice. Why doen't these dates coincide with each other, and with the solstice itself??

Reply

This may be related to an extra correction to solar time known as " equation of time ", reaching a maximum around +16 minutes in November and a minimum around 󈝺 minutes in February. A similar effect (but smaller) occurs in summer

The matter is also discussed at "Variation in time of Sunrise" http://www.physics.rutgers.edu/

Response

340. Falling off the Earth's Bottom?

My 2nd Grade grandson is full of science questions, and now one for some reason I absolutely cannot answer: If the world is round , why don't people on the "bottom" side of the world feel as if they are upside down?

I assume it has something to do with gravity, but I also wonder if it has something to do with the structure of brains and how they perceive. The problem is that even if I get a really large sphere, and put a powerful gravitational pull in the middle of it and station little metal figures all over it, from the six year old POV, the little figures on the bottom ought to feel like they are upside down, because in fact they are.

I looked through all the earth and gravity questions on your site, but couldn't find this. It may be hopelessly elementary, but I didn't know how to explain it. cheers, and thanks

Reply

If your grandson is in grade 2 at age 6, then he is doing very well indeed!

But even before that, people living by the seashore realized that ships disappeared over the horizon as if they went around a hill. First the hull could no longer be seen, then the high sails. And even after that, you could climb a hill and see it again. It only makes sense if it went "behind a hill."

Also, the stars in the sky always form the same constellations. But they are not all visible from one place. Far north, little more than half the constellations are seen. Then as you get closer to the equator, more and more appear. They still rise and set like the sun, but they are always close to the southern edge of the sky. Demonstrate to your son with a ball why this only makes sense if Earth is a ball, too.

But then, why don't we fall off? Everything suggests that the earth pulls us down towards its middle . About 350 years ago an English scientist, Isaac Newton, noted (it is said) how apples fell from the tree and wondered if the same force that pulled the apple down also held the moon close to Earth, in an orbit around it. By a calculation he showed made sense. And why doesn't the Earth run away from the sun? Same force, towards the middle of the sun. So he named that force "Universal gravitation" --universal meaning it is everywhere.

The force exists in the laboratory, too, but there, even with big balls of lead it is so weak that it can just barely be measured. Henry Cavendish first did so, some 200 years ago.

You may also ask your grandson where exactly "down" is. If the Earth is ball-shaped, there is really just one preferred direction --towards its middle!

Let your grandson keep asking good questions, and he will go far!

341. Rolling down a Slope

My daughter is a 10th grader working on her science project. I am trying to help explain why two spheres of identical size but different weights will accelerate down an inclined plane at different rates. I long ago lost my physics books and I am having a hard time putting to words (much less equations) what I think I remember about moment of inertia and angular momentum. We did a project over the weekend cutting a tennis ball and pasting lead weights to the inside to double the weight and timing their progress down an inclined plane. It was even fun to watch the numbers differ slightly when we made one ball with all of the weights at the very center (filled the ball with foam insulation first, then cored out a center hole).

Your help would be greatly appreciated. Might even help me to be the good guy

Reply

When a weight slides or rolls down a slope, all the energy comes from gravity.

Ignoring now friction, which is small in the case of a rolling object, all that energy ends up as kinetic energy . A smoothly sliding object descends with a single accelerating velocity v and its kinetic energy gain is (1/2)mv 2 , easily calculated from the loss of height.

In a rolling object the situation is more complicated: part of the energy goes into the bulk motion of the mean velocity down the slope, but part also goes into energy of rotation . (It follows that where friction can be neglected, a rolling object loses height more slowly than a sliding one!) The problem is that the kinetic energy of rotation is hard to evaluate without using integral calculus.

Consider a rotating CYLINDER. The part near the axis has very small rotation velocity and therefore negligible kinetic energy of rotation. As we increase the radius r, the velocity increases in proportion to r, until we reach the outer radius R, which moves with some velocity v and that--if the cylinder is rolling down the slope without slipping--also equals the velocity of descent (which is smaller than in the case of sliding!).

You can imagine the cylinder as being divided into narrow cylindrical layers of thickness "dr", each filling the space between distance r and r+dr , and rotating with a velocity appropriate to that distance, namely (r/R)v. That gives the layer a rotational energy proportional to [(r/R)v] 2 , and the energy is also proportional to the mass of the layer, which again is proportional to r. If we make the layers very narrow (and very numerous), the energy goes to some limit, proportional to the 4th power of R, and that is what calculus gives you.

With a rolling sphere, it is harder, because the layers have different width--widest on the axis of rotation. But this can be handled, too.

The final velocity (and time of descent) depend on the amount of energy going into rotation, and that can vary, depending how the mass is distributed. If most of it is near the axis of rotation, it rotates relatively slowly and absorbs less energy. If most of it is on the periphery (as in many flywheels) it takes a lot of the energy. That explains your experiment.

342. Pelton Wheel Efficiency

(question asked verbally by a rancher from Costa Rica)

I have a Pelton wheel turbine, receiving high-pressure water from a reservoir up on a hill. It is connected to an electric generator, which supplies power to my ranch.

When I open the casing, I see that water still rushes past the wheel with great speed, which means that the turbine is not getting all the water's energy. How can I make sure that it extracts energy most efficiently?

Reply

As described in the section on the Pelton Wheel Turbine, this turbine is most efficient when the speed u of its rotating "buckets" is half the speed v of the water jet. The velocity v depends only on the height of the reservoir and therefore (assuming the pipes are big enough) should be constant. However the velocity u depends on the electric load on the generator, which is drawn from the energy supplied by the turbine. If you overload the generator (too many lights, motors etc), u drops below optimum. If the load is light, the turbine spins easily and u is faster than the optimum.

Essentially, the efficiency of the turbine depends on the speed u of the turbine buckets. If the load is too large , u drops, and in the limit, u=0, the wheel stops and the jet is just turned around to shoot backwards, delivering zero energy. In the opposite case, if the load is essentially zero , negligible energy is produced and the jet tends to move the buckets at its own speed v. In the limit u=v, the jet hardly hits the buckets, and the stream continues forward at the same speed. In between, at u=v/2 , the maximum energy is extracted and the water dribbles down, its kinetic energy all removed.

The simplest remedy seems to be an automatic valve at the intake of the pipe high on the mountain, controlled electrically by the response of the turbine. If the water still shoots forward from the wheel, let less water in, if it is reflected backwards, let in more, and if it dribbles, the volume is right for the load. The control loop should be relatively insensitive and have a delay of a minute or two, to allow the water enough time to change its flow in the pipe and to prevent the control from reacting to any small loads switched on or off. You should be able to figure out the rest.

343. Energy loss rate of our Sun

Reply

Note: This calculation was also incorporated in the lesson plan on the Sun's energy source

The Sun gains a little mass from comets which happens to hit it, but that is a tiny amount compared to two large sources of energy loss: mass lost by the constant stream of solar wind, and mass lost by the conversion of hydrogen to helium, as part of the nuclear fusion process producing the Sun's heat. Surprisingly, the two rates are not that far apart.

If the density of the solar wind near Earth is about 10 protons per cubic cm (a bit high for an estimate, but we ignore alpha particles), moving at 400 km/sec, then a square cm at the Earth's orbit intercepts about 4 10 8 protons per sec and a square meter intercepts 4 10 12 . Assuming each proton has an energy equivalent of 10 9 ev (actually. 0.938 10 9 ) that is 4 10 21 ev (the kinetic energy of those protons is negligible by comparison). Using E=mc 2 and noting that one ev is 1.6 10 󈝿 joule, the energy-equivalent of the solar wind crossing 1 square meter per second at the Earth's orbit is 640 joule.

The solar constant , the energy of sunlight crossing 1 square meter per second, is about 1300 watt or 1300 joule per second. That energy originates in nuclear reactions in the core of the sun, converting hydrogen to helium, with the mass of the products being slightly less than those of the input ingredients. Since mass-energy is conserved, the energy-equivalent of the mass loss equals the solar constant multiplied by the area of a sphere of radius 1 AU. Per square meter of that sphere, the mass loss is equivalent to 1300 joule/sec.

Thus the sun loses twice as much mass to nuclear processes than it does to the solar wind. It seems remarkable, though, how close those numbers are

344. The Sun's Distance

I don't have any college background but am an Aviation Electronics Tech in the Navy.

However, I've alway wondered about how we know the distance of the Sun. We were told (while I was in high school science) that the earth is roughly 93 million miles from the sun. I asked my science teacher about this, and he jut gave me a dirty look. anyways, let me ask now, how did we come to that conclusion? Do we know that for sure, or is it still a hypotheses?

Reply

You asked a somewhat difficult question, but of the kind which lies at the heart of any scientific result--asking not "what is the value" (namely, 93 million miles, as many students are told to memorize) but "how do we know?" Good for you!

  "From Stargazers to Starships" traces the answer, but it isn't a simple one. First, of course, people had to realize Earth was one of the planets orbiting the Sun (section 9c) and from observations, deduce the laws which governed the motion of those planets (section 10). The orbits are all close to circles, and assume for a moment they are exactly circular and obey Kepler's 3rd law, which connects distance with the period of the orbit (one year for Earth!).

  If at any time we know the actual distance to any planet, and the relative position of Earth and that planet in their orbits at that time (i.e. the angles between the lines from each to the Sun), we can deduce the distance between those circles. From that and from Kepler's law, the radii of the circles themselves. Jean Richer in 1672 apparently was the first to try doing so ( section 10a , where the problem you ask about is discussed further). I think (not sure) he derived the slight shift of the position of Mars among the stars during a night, a shift produced because the rotation of Earth carries the observer from one side of the Earth to the other.

  More accurate values need take into account the ellipticity of the orbit, making the calculation more complicated, but the principle remains the same.

  You may also look up discussion of the first attempt to measure the distance, by Aristarchus around 200 BC--first estimating the distance to the Moon (sect. 8c) and then to the Sun (sect. 9a).

345. Why does sunlight have a continuous spectrum?

Reply

This is a bit outside my field of magnetospheric physics, but I believe the reply to #53 pretty much sums up the answer. Some other sites on this problem are

The overall conclusion is that a dense hot gas in which photons are continually absorbed and re-emitted, by atoms and ions in fast motion, or scattered by electrons, will give out a spectrum like a black body.

(If you are math oriented, it might remind you of the central limit theorem: the sum of many random variables, no matter what their characteristic distribution, tends to a bell-shaped Gaussian curve).


Why Our Analemma Looks like a Figure 8

On Monday, I posed a question to you as to why, when you photograph the Sun at the same exact time every day for a year, you get something that's shaped like a figure 8, like so:

Image credit: Tunc and Cenk Tezel.

We got a good number of thoughtful comments, many of which are on the right track, and many of which have some misconceptions. Let's clear them up, and then let's give you the explanation of what gives us our figure 8, and why other planets make other shapes.

What does the analemma look like at other places on Earth? You can see, above, that (from the ruins) the above analemma is from the Northern Hemisphere. Well, in the Southern Hemisphere (G'day to my Aussie readers!), it looks like this:

So, at the North Pole, the analemma would be completely upright (an 8 with the small loop at the top), and you'd only be able to see the top half of it. If you headed south, once you drop below the Arctic Circle, you'd be able to see the entire analemma, and it would start to tilt to one side the closer to the horizon you photographed it. By time you got down to the equator, the analemma would be completely horizontal. Then, as you continued to go south, it would continue rotating so that the small loop was beneath the large loop in the sky. Once you crossed the Antarctic Circle, the analemma, now nearly completely inverted, would start to disappear, until only the lower 50% was visible from the South Pole.

So when you do an image search and you find one that looks like this:

you know that it's photoshopped or faked, because complete, upright analemmas with other stuff on the horizon aren't completely visible from Earth! The only exception? If you photographed the Sun at exactly noon every day and never did daylight savings time. But in that case, you should get a picture of the sky, not of the horizon. (So beware of fakes!)

So, now that you know what it looks like everywhere on Earth, you're probably thinking that this analemma has something to do with the Earth's axial tilt. In fact, many of you guessed that that plays a role. You're right! You see, the Sun always traces out a nice arc through the sky, like this series of pictures taken during winter solstice from the UK:

Well, as winter transitions into summer, that arc gets higher and higher in the sky, peaking at its highest point during the summer solstice, and then declining back down to its low point as summer transitions back into the winter. The Earth's axial tilt -- responsible for this phenomenon -- explains why the Sun moves along this direction (drawn in white) of the analemma:

So on a planet like Mercury, where the axial tilt is less than one degree, the Sun's position in the sky doesn't change from day-to-day, and so an analemma on Mercury is just a single point! But something else must be going on Mars, which has almost the same axial tilt as Earth, has an analemma that looks like this:

So something must be going on that allows for variations in shape. Some planets see ellipses, some see teardrops, and some see figure 8s. Some see points, too, but they're not as interesting. (There's a list here.)

If the Earth's orbit were a perfect circle, and the Earth always moved at the same speed around the Sun, our analemma would simply be a line**, and the Sun would simply move along that line, reaching one end on the Summer Solstice and the other end on the Winter Solstice. But, no planet's orbit is a perfect circle.

Remember, if you can, Kepler's second law for planetary motion.

A line joining a planet and the sun sweeps out equal areas during equal intervals of time.

In other words, when a planet (with an elliptical orbit) is closest to the Sun (perihelion), it moves fastest. When a planet is farthest from the Sun (aphelion), it moves more slowly.

What this means is that the Earth moves different amounts through the sky as it rotates, which is important. You see, the amount of time it takes the Earth to rotate once is not 24 hours. It actually takes 23 hours, 56 minutes, and 4 seconds. Why are our days 24 hours, then? Because, on average, the Earth revolving around the Sun adds an extra 3 minutes and 56 seconds to each day. But during some days (like in March), it appears that the Sun is moving more slowly, so that 24 hours later -- what we record as a day -- the Sun has shifted its position in the sky.

This difference between the Mean Solar Time, which is our 24 hour day, and the Apparent Solar Time, which is how long it takes for the Sun to return to its same position in the sky, governs this "side-to-side" motion in the analemma. The math is given by the equation of time. But, intuitively, how does this work?

It turns out that aphelion and perihelion are close to the solstices on Earth. During these times, a day is actually very close to 24 hours. When the Earth moves from aphelion toward perihelion (when we're experiencing the autumnal equinox in the Northern hemisphere), the Sun appears to move quickly, and so it reaches its apex in the sky at times slightly earlier than during the solstices. Conversely, when the Earth moves from perihelion to aphelion (during the months of February and March, for example), the Sun appears to move more slowly, and so reaches its apex at slightly later times than normal.

We call these two situations a "fast Sun" and a "slow Sun". If the y-axis of the analemma was due to the Earth's axial tilt, then the x-axis comes from the Sun appearing fast or slow:

So why is Earth a figure 8 and Mars a teardrop? Because Mars' perihelion and aphelion line up close to Mars' equinoxes, rather than the solstices like it does on Earth. Know what this means? As the Earth's equinoxes precess (which they do over a time period of 26,000 years), the shape of our analemma will change. So enjoy the figure 8 now, while we have it!

Update: An astute commenter has pointed out that the Earth's axial tilt also contributes to the Sun's apparent motion in not just the up-down direction, but also in the "side-to-side" motion. I've managed to find an animated image that shows:

  1. the effect of eccentricity (what I talked about above),
  2. the effect of axial tilt (something that most planets have),
  3. the combined effects of both of these (which gives us our equation of time), and
  4. the overall path of the analemma, which aligns neatly with the equation of time.

So, if one of these (like eccentricity) always dominates the other (as is the case on Mars), we get a teardrop. If one of them (like eccentricity) is significant and the other is practically zero (as is the case on Jupiter, with a 3 degree tilt only), you get something much closer to an ellipse. And if both are important enough that sometimes eccentricity dominates and sometimes axial tilt dominates (as is the case for Earth, with a tiny eccentricity, and Uranus, with a huge 88 degree axial tilt), you get a figure 8!

** -- Also, note that what I wrote up top about the analemma simply moving up and down in a straight line is also incorrect. The Earth's axial tilt (also called obliquity) would still be present, and would still contribute to the side-to-side motion of the Sun in the sky, even if the orbit were a perfect circle.

So you see, this deceptively simple question is actually incredibly complex, and I make mistakes sometimes!


Reincarnation and the Holy Name

In recent years interest in reincarnation has grown, with new advocates, theories, and discoveries. Testimonies by persons who have returned from the verge of death after supposedly glimpsing the hereafter have intrigued modern parapsychologists, as well as researchers like Elizabeth Kubler-Ross, author of On Death and Dying, and Raymond Moody, author of Life After Life and other bestsellers.


The original source—books on reincarnation, however, are the Sanskrit Vedic, literatures. The Srimad- Bhagavatam, for example, gives a fascinating account of the near-death experience of a man named Ajamila. Unlike modern investigations, the case of Ajamila lets us study the near-death experience not from the viewpoint of the dying person but from the viewpoint of higher beings present at the time of the soul’s passing out of the body.

Srimad-Bhagavatam relates how the messengers of Death and the messengers of Lord Vishnu, the Supreme Personality of Godhead, disagreed over where Ajamila should reincarnate in his next life. Being deathless, the atma, or self, must take birth in another body when the present body ceases to function. And that next body is determined by one’s individual karma: “As you sow, so shall you reap.”

In the case of Ajamila, the messengers of Death wanted to drag the soul to hell because of his life of sin. Although Ajamila lay in a coma, he was conscious of the messengers of Death preparing to transfer him to the lower regions. But suddenly the beautiful, effulgent messengers of, Vishnu arrived and intervened. The messengers of the Lord said the messengers of Death had misjudged the soul of Ajamila and had no right to take him.

Incensed, the messengers of Death explained why Ajamila should be taken and punished. Judging a person’s karma, they said, is a relatively simple thing. At the time of death, when a soul is ready to enter another body, the superintendent of Death arranges for a future body in accordance with the particular soul’s past sinful and pious acts. Because Ajamila had led a sinful life, he was now due to be punished.

The messengers of Death gave an analogy: As springtime in the present indicates the nature of springs in the past and future, so this present life of happiness or distress indicates one’s activities in the past, and one’s present activities are an index of one’s future incarnations. In other words, on the basis of the activities a person performs in his present life, the higher authorities determine his destiny in the next life.

Since most people incur at least some bad karma, it is the duty of the messengers of Death to transfer them to a lower position. Most people act without any understanding of the law of karma and thus commit all kinds of abominable acts for the pleasure of the present body. They do not know that their present suffering is a result of past sins, nor are they able to understand that their present sins will cause them future suffering. Acting in the darkness of ignorance, most people are unable to know their past or future lives. And even when they hear from the Vedic literature about transmigration of the soul and the law of karma, they refuse to accept that there is anything beyond this present life of sense .gratification.

Such an ignorant person was Ajamila. And because of his life of sin, the messengers of Death saw no reason why the messengers of the Lord should obstruct their work of awarding him his just karma.

The messengers of Lord Vishnu, however, asked the messengers of Death on what basis they had judged Ajamila. The messengers of Death replied that they had judged him according to the religious scriptures. They then read a long list of criminal, violent, irresponsible, irreligious, and perverted acts Ajamila had committed. At this, the messengers of Vishnu admitted that hellish punishment would ordinarily await such a sinner but in the case of Ajamila, this did not apply.

The extraordinary circumstance in Ajamila’s case was that at the last moment of his life he had called out the name of God, Narayana. Although he was not thinking of God but of his son Narayana, he had nevertheless called out, “Narayana!” This had neutralized all Ajamila’s bad karma and had saved him.

The messengers of Vishnu explained that Ajamila’s uttering the name Narayana had absolved him of all his sins—not only those of his present life but those of millions of past lives. He had chanted without offense and was therefore purified and eligible for liberation. The messengers of Vishnu explained that even if a person chants the name of God indirectly (to indicate something else), jokingly, for musical entertainment, or even neglectfully, the holy name will still free him from the reactions of all sins. No matter how sinful a person may be, the holy name of God has the power to absolve him and save him from hellish punishment.

Unable to oppose the higher authority, the messengers of Death released Ajamila. The supernatural beings vanished, Ajamila awakened from his coma, and by the grace of the Lord he was able to spend his remaining days in devotional meditation on the Supreme Personality of Godhead.

This account from Srimad-Bhagavatam gives us valuable information about the soul, the next life, the laws of karma, and the potency of the holy name of the Lord. For those interested in reincarnation, the Vedic literatures are worth investigating. Rather than limit oneself to empirical data from modern researchers, one should consult Srimad-Bhagavatam and Bhagavad-gita for a clear understanding of reincarnation and the specific importance and responsibility of the human form of life. As Socrates said, “The unexamined life is not worth living.” And an essential part of one’s life to examine is one’s death. What happens at this critical time? Is there a next life? If so, how can we assure the best next life for ourselves? Certainly any introspective, openminded investigation into the subject of reincarnation would be incomplete without careful study of Vedic writings like Srimad-Bhagavatam and Bhagavad- gita.


7 Irregularities That Supposedly Suggest Earth’s Moon Was Engineered

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Even though the Apollo missions brought back to Earth huge amounts of data about the moon, it has remained an enigma for astronomers and scientists alike. Dr. Robert Jastrow, the first president of NASA’s Commission of Lunar Exploration called the moon “the Rosetta Stone of the planets.”

But what is it about the moon that fascinates everyone?

Well, there are many people who firmly believe that Earth’s moon is actually a terraformed and engineered piece of hardware that has a 3-mile thick outer layer of dust and rocks. Beneath this layer, it is believed that the moon has a solid shell of around 20 miles made of highly resistant materials such as titanium, uranium 236, neptunium 237.

Definitely, elements that you would not expect to find “inside” the moon.

There are many UFOlogists around the world who speculate that the moon is actually a giant base where extraterrestrials survey mankind’s progress.

There are so many mysteries surrounding Earth’s moon that there are those who have proposed that the moon could be something entirely different.

Robin Brett, a scientist from NASA stated, “It seems easier to explain the non-existence of the Moon than its existence.

Here are 7 Irregularities that suggest Earth’s Moon was engineered and might be a giant hollow base:

1) The Moon seems engineered. On November of 1969, NASA intentionally crashed a lunar module that caused an impact equivalent to one ton of TNT on the Moon. The shock waves built up and NASA scientists listened to what was happening on the Moon. Strangely, after impact, NASA scientists said that the Moon rang like a bell and the reverberation continued for thirty minutes. According to Ken Johnson, supervisor of the data and photo control department, the Moon not only rang like a bell but the whole Moon “wobbled” in such a precise way that it was “almost as though it had gigantic hydraulic damper struts inside it.

2) The Moon has elements it should not have. In the 1970’s, Mikhail Vasin and Alexander Shcerbakov from the Soviet Academy of Science wrote an article called: “Is the Moon the creation of Alien Intelligence?” It was a very interesting article that asked some important questions. How is it possible that the surface of the moon is so hard and why does it contain minerals like Titanium? Mysteriously there are some lunar rocks that have been found to contain PROCESSED METALS such as Brass, the elements of Uranium 236 and Neptunium 237 that have NEVER been found to occur naturally. Yet there are traces of them on the Moon. Uranium 236 is a radioactive nuclear waste which is found in spent nuclear and reprocessed Uranium. More interestingly, Neptunium 237 is a radioactive metallic element and a by-product of nuclear reactors and the production of Plutonium. You have to ask the question: What is happening on Earth’s Moon? From where do these elements and minerals come from?

3) Earth’s Moon does not have a solid core like every other planetary object. Researchers are nearly 100 percent sure that the Moon is, in fact, hollow or has a very low-intensity interior. Strangely, the Moon’s concentration of mass is located at a series of points just below the surface.

4) The Moon is older than Earth. Our Moon is unlike any other satellite discovered in the known universe. Researchers know the Moon is 4.6 billion years old and that raises a lot of questions. This means that the moon is older than the Earth by nearly 800,000 years according to scientists.

5) Incredible orbit. Earth’s moon is the only moon in the solar system that has a stationary, nearly “perfect” circular orbit. It’s a fact that the Moon does not spin like a natural celestial body. In other words, our Moon does not share any characteristics with other moons found in our Solar System. If that isn’t strange enough, consider that from any point on the surface of our planet only one side of the Moon is visible. What is the moon hiding?

6) Lunar rocks and titanium. There are some lunar rocks that have been found to contain ten times more titanium than “titanium rich” rocks on planet Earth. Here on Earth, we use Titanium in supersonic jets, deep diving submarines and spacecraft. It’s unexplainable. Dr Harold Urey, Nobel Prize winner for Chemistry said he was “terribly puzzled by the rocks astronauts found on the moon and their Titanium content. The samples were unimaginable and mind-blowing since researchers could not account for the presence of Titanium.

7) Precise position. If all of the above points do not get you to think differently about the Earth’s moon here are some more interesting things about the Moon. What is keeping the moon in its nearly perfect position? The moon has a precise altitude, course and speed, allowing it to “function” properly in regards of planet Earth.

Simply put the Moon should not be where it is currently. Everything points to the possibility that Earth’s moon was in fact placed into its current orbit in the distant past. The Moon’s unnatural orbit and irregular composition raise hundreds of questions that neither NASA scientists, astronomers or geologists are able to answer today. Despite all efforts to understand Earth’s “natural” satellite, the truth is that we have very little information about the Moon’s origin and purpose. What do you think the moon is? A nearly perfect natural occurrence? Or do the Moon’s origins surpass human understanding?


Why aren't there eclipses every month?

July 22, 2009 Solar Eclipse. Credit: Bill Fish

If the sun, Earth and moon are lined up, shouldn't we get a lunar and solar eclipse every month? Clearly, we don't, but why not?

Coincidences happen all the time. Right, Universe? One of the most amazing is that moon and the sun appear to be almost exactly the same size in the sky and they're both the size of your pinky fingernail held at arm's length. These coincidences just keep piling up. Thanks Universe?

There are two kinds of eclipses: solar and lunar. Well, there's a third kind, but we'd best not think about that.

A solar eclipse occurs when the moon passes in between the Earth and sun, casting a shadow down on the surface of our planet. If you're in the path of the shadow, the moon destroys the sun. No, wait, I mean the moon blocks the sun briefly.

A lunar eclipse happens when the moon passes through the Earth's shadow. We see one limb of the moon darken until the entire thing is in shadow.

You've got the sun, Earth and moon all in a line. Where they're like this, it's a solar eclipse, and when they're like this, it's a lunar eclipse.

If the moon takes about a month to orbit the Earth, shouldn't we get an eclipse every two weeks? First a solar eclipse, and then two weeks later, lunar eclipse, back and forth? And occasionally a total one of the heart? But we don't get them every month, in fact, it can take months and months between eclipses of any kind.

If the sun, Earth and moon were truly lined up perfect, this would be the case. But the reality is that they're not lined up. The moon is actually on an inclined plane to the Earth.

Imagine the solar system is a flat disk, like a DVD. You kids still know what those are, right? This is the plane of the ecliptic, and all of the planets are arranged in that disk.

But the moon is on another disk, which is inclined at an angle of 5.14 degrees. So, if you follow the orbit of the moon as it goes around the Earth, sometimes it's above the plane of the ecliptic and sometimes it's below. So the shadow cast by the moon misses the Earth, or the shadow cast by the Earth misses the moon.

But other times, the sun, moon and Earth are aligned, and we get eclipses. In fact, eclipses tend to come in pairs, with a solar eclipse followed by a lunar eclipse, because everything is nicely aligned.

The geometry that creates a total lunar eclipse. Credit: NASA

Wondering why the moon turns red during a lunar eclipse? It's the same reason we see red sunsets here on Earth – the atmosphere filters out the green to violet range of the spectrum, letting the red light pass through.

The Earth's atmosphere refracts the sunlight so that it's bent slightly, and can illuminate the moon during the greatest eclipse. It's an eerie sight, and well worth hanging around outside to watch it happen. We just had recently had a total lunar eclipse, did you get a chance to see it? Wasn't it awesome?

Don't forget about the total solar eclipse that's going to be happening in August, 2017. It's going to cross the United States from Oregon to Tennessee and should be perfect viewing for millions of people in North America. We've already got our road trip planned out.

Lunar Eclipse from New Jersey 12-21-2010. Credit: Robert Vanderbei


Watch the video: Το ηλιακό σύστημα (February 2023).