# Size of Sun as seen on Pluto

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What would be the size of Sun as seen from surface of Pluto? It obviously depends on the distance and one would expect it to be very small. However, due to different optical phenomenon, the size of Sun seen on surface of Pluto could be larger. Can it can be estimated from the annular eclipse Sun-Pluto eclipse picture or other pictures taken by New Horizons probe ? Thanks for your insight.

The picture is also posted on this forum question.

Edit: From the answer given by @Conrad Turner below, Sun should be just pin point at that distance. However, when seen by New Horizons probe from behind Pluto, its size is seen to be almost as large as Pluto. The atmosphere of Pluto must be bending the light so much to make this happen.

Update: This picture was taken from a distance of 2 million km from Pluto. At this distance Sun will have an angular size of 0'49" (distance 5804550000 km, diameter 1392000 km), while Pluto has the angular size of 4'5" (distance 2000000 km, diameter 2372 km). However, the picture is showing Sun to have an angular size slightly larger than Pluto (or nearly the same size). Hence, high refraction or scattering or some other phenomenon are at work here.

Considering sizes and distances, if atmosphere of Pluto does nothing to light, the ring of Sun should have been visible around Pluto once probe was 10 million km beyond Pluto. However, the ring of Sun is seen in this picture when probe is only 2 million km beyond Pluto. The scientists must have known the optical characteristics of Pluto's atmosphere and hence they got the probe early in proper position to take this picture.

The atmoshere on Pluto will have very little effect on the apparent size of the Sun, It is negligable compared to that on Earth ($sim 0.3-1.0 mbox{ Pa}$ compared to $sim 10^5 mbox{ Pa}$, approximately the air pressure at 100 km on the Earth).

Now angular size is proportional to the ratio of the objects diameter to its range. At the Earth's distance ($1 mbox{ AU}$) the Sun subtends about $30'$ of arc. So at Pluto (as it is at a distance from the Sun of between $26.7 mbox{AU}$ and $49.3 mbox{AU}$ depending on where in its orbit it is) the Sun will subtend between $30/26.7approx 1.1'$ and $30/49.3approx 0.6'$ of arc. Which is close to the resolution of the human eye corresponding to 20/20 vision.

New Horizons encountered Pluto when it was about 33.5 au from the Sun.

The solar diameter is $1.39 imes 10^{9}$ m.

An astronomical unit is $1.496 imes 10^{11}$ m.

The angle subtended by the Sun at Pluto (in radians) is the ratio of the diameter of the Sun to its distance from Pluto. Converting to degrees, minutes, seconds, gives 0.01589 degrees, or 0.95 arcminutes. The atmosphere of Pluto is way too thin (much thinner than the Earth's) to make any significant difference to this at all.

The diameter of Pluto is $2.37 imes 10^6$ m. In order for Pluto and the Sun to subtend a similar angle in an image, you just need to move far enough away from Pluto to make the same angle.

Thus $$D_P/R_P simeq (D_P + D_{PS})/R_S,$$ where $D_P$ would be the distance from New Horizons to Pluto, $D_{PS}$ the distance from Pluto to the Sun, $R_P$ the radius of Pluto and $R_S$ the radius of the Sun. Because I will show that $D_P ll D_{PS}$ we can say that $$D_P simeq frac{R_P}{R_S} D_{PS} simeq 0.057 au$$.

New Horizons is travelling at about 14.5 km/s. So a simple bit of Maths tells us it would takes about 6.5 days to get into this position.

The picture I think you are talking about was taken on July 15th 2015 only 7 hours after the flyby. The Sun and Pluto do not share the same angular size in this photo. You are not seeing the Sun at all in this picture. It is eclipsed by Pluto completely. All that is being seen is sunlight refracted (or more likely scattered) through Pluto's atmosphere. The Sun acts more-or-less like a point source of light at more-or-less infinite distance.

The angle that the light needs to deviate in order to be seen through the atmosphere is about $0.19$ degrees (the half-angle subtended by Pluto at the Sun [negligible] plus the angle subtended by Pluto at New Horizons after 7 hours). If we were to model the atmosphere as a glass prism. The deviation angle is $delta simeq (n-1) alpha$, where $n$ is the refractive index and $alpha$ is the opening angle of the prism. If you let $alpha$ be quite large (let's say 1 radian) to simulate rays coming through the atmosphere, then $n = 1 + delta$, where $delta$ is in radians. So $nsimeq 1.003$. NB: This assumes there is no scattering in Pluto's atmosphere, but I suspect that is not true and that my estimate of the refractive index is a large overestimate. In fact I'm sure it is an overestimate because this is larger than the refractive index of air on Earth. i.e. I am sure that what we are seeing is scattered light, not refracted light.

## Scaling the solar system

Every now and again, in interviews and on social media, I'm asked an interesting question: If there was one thing you wish people understood better about astronomy, what would it be?

Things in space are very, very, very far away. The closest natural object to us, the Moon, is nearly 400,000 kilometers from Earth. The Saturn V rocket, still the most powerful rocket ever to be successfully used, took over three days to fling astronauts to the Moon. Three days of crossing nothing but an empty gulf.

And that's just the Moon. Getting to Mars and Venus, the two closest planets, takes months of travel. Jupiter is 600 million km away at its closest. Did you know that Saturn is twice as far as Jupiter is, and Uranus twice as far as Saturn?

It's incredibly difficult to wrap your head around these scales. Worse, analogies tend to fall flat. Example: It would take nearly two centuries to drive to the Sun if there were a road to it (and your windows were shut really tightly). Does that visualize how far that really is?

It gets worse if you try to use planetary sizes for comparison. Planets are tiny compared to the distances separating them. Earth is just under 13,000 kilometers across, but it's 150 million km from the Sun. Over 11,000 Earths could fit between Earth and the Sun. Oof.

But there is a helpful model: a scale model. Across the world there are quite a few models of the solar system, and they tend to be pretty big. I was reminded of this when I saw this video, where a filmmaker, Wylie Overstreet, created a scale model of the solar system in the Nevada desert.

Impressive! And it reminded me of something …

I used to be part of an education and outreach group at Sonoma State University. We had NASA grants to put together educational materials about basic math and science. My work partner Sarah Silva and I would go to classrooms and talk to kids, and one day we decided to tackle this issue. We took an older exercise, called the Solar System Rope (you can find many variations of it online), and modified it to suit our needs. Basically, it uses a 20- or 30-meter rope that represents the distance from the Sun to Neptune (or Pluto). We marked it to show where the planets were. We'd have one kid be the Sun and hold one end, then another be Mercury (we printed out pictures of the planets and Sun for them to hold, too) and hold the rope at the right place, and so on.

When they were done, it was pretty amazing: Mercury, Venus, Earth, and Mars are all cramped and huddled within a meter or so of the Sun, but the outer planets were waaaaaay far away. It did a wonderful job impressing on them how big the solar system was, and why it took the New Horizons probe nearly a decade to get to Pluto.

But when we were setting it up the first time, I constantly found myself doing the math over and again to get the model scale right. If the Sun was one centimeter across, how big was the solar system? What if the Earth were that big?

After a few minutes of that I got tired of it, so I did something unprecedented * : I created a spreadsheet with all the needed numbers in it, coding it with the sizes of the planets and Sun, their distances, and so on. All you have to do is put in the size of the Sun you want and it calculates the size of the model.

Using this you can create a scale model of the solar system to any specification you want. And now, out of an act of pure magnanimity, I make this available to you for free! I put it on Google Docs for you to poke at it's read-only there, but you can download it and adapt it to your heart's desire.

A snapshot of the solar system scale model spreadsheet using the default size of the Sun of 100 cm (1 meter). Credit: Phil Plait

You don't need to know much about spreadsheets to work it. The first column is the name of the object. The second and third are the radii and diameters in kilometers. In the third column, I divided the diameters by the Sun’s diameter, so now you have them all in terms of the size of the Sun. The Sun is one Sun diameter across, the Earth is 0.00918. The next column is the distance from the Sun in km, then that same distance divided by the Sun's diameter.

The seventh column is the size of the Sun in your scale model. The default is 100 centimeters (1 meter). The next column then scales the planets’ sizes to that, and the final column is the distance your planet is from the Sun. Easy peasey † .

If you want a smaller Sun, replace the 100 cm in column G with something smaller. Note how much smaller the scale model gets. You can adjust it to get a planet size you want as well if you want the Earth to be 30 cm across, say, play with the Sun size until it works out that way (answer: The Sun would be about 3300 cm across 33 meters. That's HUGE).

Now go back and check the solar system scale model video again. At the five-minute mark, the Sun rises and they compare the size of the Sun and their model Sun as seen from the model Earth … and it works! Their Sun was over a meter wide, and Earth was somewhat bigger than a centimeter and 176 meters from the Sun. If you check my spreadsheet, that matches.

One more thing: Out of curiosity, I added the Kuiper Belt to the spreadsheet and even the distance to Alpha Centauri, the nearest star system. If you scale the model to something reasonable, how far away is Alpha Cen? A loooooong way. So far that even with a scale model it's hard to keep that distance on the Earth itself: If the Sun is a meter across, Alpha Cen is nearly 30,000 km away! That’s nearly as high as actual geosynchronous satellites above the Earth ** .

Amazing. If you're an educator, I really recommend the rope activity. It's interesting, it gets the kids up and outside (or in a very long hallway), it's kinesthetic, and most of all it's fun! The kids really will enjoy being a part of it.

And that's the point, isn't it? We're all a part of the solar system, and we should take a minute to appreciate that. We may be small — too small to see on the scale model! — but the fact that we can figure all this out makes us big.

* Unprecedented for me, that is. I hate spreadsheets. Hate them. Hate. I wish I could banish them all to the end of the solar system rope, except not to scale.

** D'oh! I did the conversion incorrectly in the original text, saying it was 300,000 km. See? This is why I hate spreadsheets. :)

## BAFact math: how big does the Sun look from Pluto?

[On January 4, 2012, I started a new features: BAFacts, where I write an astronomy/space fact that is short enough to be tweeted. A lot of them reference older posts, but some of the facts need a little mathematical explanation. When that happens I’ll write a post like this one that does the math so you can see the numbers for yourself. Why? Because MATH!]

From Pluto, the Sun is so far away it would appear to be a point in the sky like a star, though an incredibly bright one.

Yesterday, I showed how the Sun would still be painfully bright even from Pluto, far brighter than the full Moon looks from here on Earth. But how big would it look in the sky?

It turns out, that math is even easier than it was to find the brightness! The size of an object on the sky depends on how big it really is, physically, and how far away it is. If you double the distance to an object, it will appear half the size. Easy peasy * .

So, as I established yesterday, on average Pluto is about 39 times farther from the Sun than the Earth, so if you were standing on Pluto (hopefully, in a well-heated and insulated spacesuit!) the Sun would appear 1/39th as big, or 0.026 times as big as it does from Earth.

What would that look like?

Well, the size of the Sun in the sky from Earth is about a half a degree – remember, there are 360° in a circle. So from the horizon to the zenith is 90°, and your outstretched fist is very roughly 10°. The Sun is about 0.5°, so you can block it with a single finger held at arm’s length.

From Pluto, though, it’s far smaller: less than 1 arcminute in size (a degree is divided into 60 arcminutes, so from Earth the Sun is about 30 arcmin across). That brings up an interesting point: the smallest size the human eye can easily resolve is something about an arcminute across. Anything smaller than that looks like a dot.

So from Pluto, the Sun would look like a star – that is, a point of light – albeit an intensely bright one. Looking at it would certainly be painful, and probably make your eyes tear up.

But wait! I also mentioned yesterday that Pluto’s orbit is an ellipse, and it goes from 4.4 billion to 7.3 billion km from the Sun. That’s a factor of 29 to 49 times the Earths distance from the Sun. So that shrinks the size of the Sun accordingly. When Pluto is farthest from the Sun (called aphelion) the Sun is far less than an arcminute in size, and looks like a dot. When Pluto is closest to the Sun (perihelion) it will actually be just about one arcminute in diameter. Someone with sharp eyes might be able to perceive it as a disk rather than a point of light… though that would still be really tough to do, because the Sun’s still so bright. If you had a filter in your spacesuit visor you’d be able to see the disk of the Sun.

If you’re curious, blogger Burton MacKenzie made a simple diagram showing how big the Sun is from each of the planets (thumbnail shown here click to ensolarnate). He didn’t put Pluto on it, but from there the Sun would look even smaller on average than it does from Neptune.

Never forget: the solar system is big! The New Horizons probe was launched in early 2006, is screaming across the solar system at 15 km/sec (fast enough to cross the entire US in about 5 minutes!) but still won’t pass Pluto until mid-2015.

Space is deep, vast, and empty. From far enough away, even the Sun itself would be dimmed to invisibility. If there’s a life lesson in there somewhere, feel free to find it.

Image credit: ESO, annotated by me

* Well, almost easy peasy. This only works well if the object is far enough away that it appears small to you. There’s actually a trigonometric formula to do this exactly, but it hardly matters for something the size of the Sun, even at Mercury’s distance, saying its apparent size changes linearly with distance is OK.

## Swarms of Pluto-size objects kick up dust around adolescent Sun-like star

An artist’s impression of the debris disc around Sun-like star HD 107146. This adolescent star system shows signs that in its outer reaches, swarms of Pluto-size objects are jostling nearby smaller objects, causing them to collide and “kick up” considerable dust. Image credit: A. Angelich (NRAO/AUI/NSF)

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) may have detected the dusty hallmarks of an entire family of Pluto-size objects swarming around an adolescent version of our own Sun.

By making detailed observations of the protoplanetary disc surrounding the star known as HD 107146, the astronomers detected an unexpected increase in the concentration of millimetre-size dust grains in the disc’s outer reaches. This surprising increase, which begins remarkably far — about 13 billion kilometres — from the host star, may be the result of Pluto-size planetesimals stirring up the region, causing smaller objects to collide and blast themselves apart.

ALMA image of the dust surrounding the star HD 107146. Dust in the outer reaches of the disc is thicker than in the inner regions, suggesting that a swarm of Pluto-size planetesimals is causing smaller objects to smash together. The dark ring-like structure in the middle portion of the disc may be evidence of a gap where a planet is sweeping its orbit clear of dust. Image credit: L. Ricci, ALMA (NRAO/NAOJ/ESO) B. Saxton (NRAO/AUI/NSF)

Dust in debris discs typically comes from material left over from the formation of planets. Very early in the lifespan of the disc, this dust is continuously replenished by collisions of larger bodies, such as comets and asteroids. In mature solar systems with fully formed planets, comparatively little dust remains. In between these two ages — when a solar system is in its awkward teenage years — certain models predict that the concentration of dust would be much denser in the most distant regions of the disc. This is precisely what ALMA has found.

“The dust in HD 107146 reveals this very interesting feature — it gets thicker in the very distant outer reaches of the star’s disc,” said Luca Ricci, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author on a paper accepted for publication in the Astrophysical Journal. At the time of the observations, Ricci was with the California Institute of Technology in Pasadena.

“The surprising aspect is that this is the opposite of what we see in younger primordial discs where the dust is denser near the star. It is possible that we caught this particular debris disc at a stage in which Pluto-size planetesimals are forming right now in the outer disc while other Pluto-size bodies have already formed closer to the star,” said Ricci.

According to current computer models, the observation that the density of dust is higher in the outer regions of the disc can only be explained by the presence of recently formed Pluto-size bodies. Their gravity would disturb smaller planetesimals, causing more frequent collisions that generate the dust ALMA sees.

The new ALMA data also hint at another intriguing feature in the outer reaches of the disc: a possible “dip” or depression in the dust about 1.2 billion kilometres wide, beginning approximately 2.5 times the distance of the Sun to Neptune from the central star. Though only suggested in these preliminary observations, this depression could be a gap in the disc, which would be indicative of an Earth-mass planet sweeping the area clear of debris. Such a feature would have important implications for the possible planet-like inhabitants of this disk and may suggest that Earth-size planets could form in an entirely new range of orbits than have ever been seen before.

The star HD 107146 is of particular interest to astronomers because it is in many ways a younger version of our own Sun. It also represents a period of transition from a solar system’s early life to its more mature, final stages where planets have finished forming and have settled into their billions-of-years-long lifetimes in orbit around their host star.

“This system offers us the chance to study an intriguing time around a young, Sun-like star,” said ALMA Deputy Director and coauthor Stuartt Corder. “We are possibly looking back in time here, back to when the Sun was about 2 percent of its current age.”

The star HD 107146 is located approximately 90 light-years from Earth in the direction of the constellation Coma Berenices. It is approximately 100 million years old. Further observations with ALMA’s new long-baseline, high-resolution capabilities will shed more light on the dynamics and composition of this intriguing object.

Pluto has very thin atmosphere with a surface pressure 100,000 times less than that on Earth. It is composed of 98% nitrogen and small amounts of methane and carbon monoxide – much like the atmosphere of Neptune's moon Triton.

Pluto's atmosphere gradually freezes and collects on the surface as the planet moves away from the Sun. Yet, interestingly, observations have shown that between 1989, when Pluto was at perihelion, and 2002, the atmospheric pressure increased threefold. The explanation probably has to do with the fact that materials take time to warm up and cool off, which is why the hottest part of the day on Earth, for example, is usually around 2 or 3 pm rather than local noon, when sunlight is the most intense. The fact that Pluto's atmosphere is still building up rather than freezing out, as many scientists expected, is good news from the standpoint of what the New Horizons probe, launched toward Pluto in 2006, might learn of this enigmatic world.

## I turned the Pluto image into an 8K wallpaper for those that want it.

Simple take a short little 34 au trip to space for some more pics with cool filters.

You just mantra the word "enhance" while keying your fingers over the keyboard.

You hero, you. Saved, and set as background!

Someone linked this recently so I know this is coming out of nowhere, but I totally read this comment in a William Shatner pace for some reason.

How come I see boxes of a lighter black around it? the black isnt totally black.

I do too on an MacBook Air screen. OP's brightness is probably too low or he has glare on his screen if he missed them. I've even seen a white pixel somewhere, which would look like a dead pixel on an 8K screen.

If anyone wants to correct this, open the original image in Photoshop, select Pluto precisely using the round selection tool while pressing shift to make a perfect circle, copy the selection, create a new document, paste the selection, select the background layer, expand the size of the canvas to 7680 by 4320 pixels in all directions, paint the background black, flatten the image, use a large spot healing brush tool to blend the border between the black background and the small part of the planet that isn't lit by the sun, save the file in JPG without compressing it visibly and voilà!

Pluto is about one-sixth the width of Earth, at just 1,420 miles in diameter.

It is far smaller even than Earth’s moon, and 30 times smaller than Mercury, the smallest planet in our solar system.

New Horizons identified Pluto as the largest object in the Kuiper belt and, crucially, larger than Eris.

Charon is the largest of Pluto’s five moons, and is half the size of Pluto, so they are sometimes together called the double planet.

### SAVE OURSELVES

New Horizons showed Pluto not only to be larger than previously thought, but to have a thicker atmosphere and to be more geologically dynamic.

Pluto’s surface has broad plains and valleys, several kilometre-high mountain ranges, even volcanos.

The range of chemicals and colours found on Pluto differentiates it from others in the Kuiper belt, and some regions of Pluto appear new whereas others are very old.

## Models in Astronomy

Models of the solar system show the positions of the planets and their moons as they orbit the Sun. While these models are useful in comparing the sizes of the planets and moons, it is difficult to build them to scale. Distances between planets are huge compared to the size of the planets.

Earth

Moon

If you stood by Epcot's geosphere holding a baseball in one hand and a globe in the other, you would see a scale model of the differences in the sizes of the real objects. However, it wouldn't be a scale model for the distances between the objects. To make a scale model for both the sizes and the distances, you would need to stand 3 miles (4.8 kilometers) away from the geosphere, holding the globe, and then you would need a friend to stand 40 feet (12 meters) from you, holding a baseball.

Think of the globe in your classroom. Most classroom globes are 16 inches (41 centimeters) in diameter. If you made a model of the solar system and used the classroom globe as your Earth, our Moon would be about the size of a baseball, with a diameter of 4 inches (10 centimeters).

Remaining consistent in your scale, the Sun would have to be 146 feet (44.6 meters) in diameter, a little smaller than the diameter of the geosphere at Epcot Center in Florida.

Sun

Another model, using a different scale, would make it easier to view the distance from the Sun to Pluto all at once. If you placed the Sun on one goal line of a football field, Pluto's position would be on the opposite goal line, 100 yards (91.4 meters) away. Earth would sit 2.5 yards (2.3 meters) away from the Sun. This model gives a better understanding of the relative distance from the Sun to each planet, ending at Pluto. But the reduced scale of this model makes it necessary to reduce the size of the Sun and planets as well. No longer the size of a classroom globe, in this scale the Earth would have a diameter of 0.008 inch (0.2 millimeters). Even the Sun would be smaller than one inch wide. Its diameter would measure 0.85 inches (21.5 millimeters).

Sitting in the stands of the football stadium, we wouldn't even be able to see the Sun and the tiny planets. But if we placed a stake in the ground at each planet's position, we could at least get an idea of where they would be, and we would have a better understanding of the immensity of our solar system.

AU stands for astronomical unit. It is the distance from the center of the Earth to the center of the Sun. In this scale model, it is 2.5 yards (2.3 meters), but a true AU is 93 million miles (149.6 million kilomters).

## Scaling the Solar System

Most of us have seen diagrams such as the classroom picture above, showing the relative size of the planets. Note that they can never get more than a small arc of the entire Sun’s orb in these pictures. If all of the objects were spaced out proportionally, at this scale how big would the Sun appear? How far away would Earth and all the planets be?

This exercise isn’t just for students. I knew the Sun was about 93 million miles away. I knew the Earth was about 8,000 miles in diameter. But I was stupefied to learn the Sun is the better part of a million miles in diameter: 864,936 miles is the published figure I found.

All right then, what would all this “look like”, scaled down to human scale? I need something I can “see”. After playing around with the numbers, I worked up a Solar System Spreadsheet to help do the math and tabulate the results. If you have room on your monitor, go ahead and open up the spreadsheet, and follow along with the narrative.

Size: To make everything “fit” into an imaginary layout box in the parking lot visible below our apartment, I chose a scale which makes the Earth the size of a large pea, or about 1/4 inch diameter. On this scale, the Moon is 0.07 inches in diameter, which is about the diameter of a grain of rice. Our Sun becomes just over 27 inches in diameter, about the size of a beach ball.

Distance: orbiting our pea-size Earth, we place our tiny Moon 7″ from the Earth. The Sun scales out to 244 feet away, in the far corner of the parking lot. There is still not enough room on our apartment grounds to fit any of the other more distant scaled panets. Mars would be out on the Boulevard somewhere. Tiny Pluto would be nearly two miles distant.

Imagine our Sun as a tiny hydrogen bomb. On this scale, imagine its mushroom cloud to be the size of our beachball. There is sufficient energy at 244 feet (about 30 watts per square meter) to provide heat, light and energy to keep the pea-sized Earth model in a comfy “habitable zone”.

Weight: These calculations were tougher. I had to get the actual densities (kilograms per cubic meter) of the Moon, Earth and Sun. I applied these to the scale volumes of our models. The Sun, at the center of its core, is 8 times as dense as gold (due to the tremendous gravitational compression), but quite gaseous in its hot outer photosphere. (The depth of just the photosphere is about half the distance from Earth to Moon). The Sun has an average density somewhat greater than water, but less than rock.

Note that the actual mass of the Sun is 1.98892 time ten to the thirtieth power kilograms: that’s two, followed by 30 zeroes, kg.

If my calculations are correct, our beachball Sun would weigh 403 pounds. The Earth would weigh as much as, well, a pea-sized pebble.

So there you have it. If you were standing in the parking lot corner with our model beachball sun, you would not be able to see our model Earth with the naked eye. You might be able to pick it out with binoculars — unless it was camoflaged in the landscaping, as real planets are camoflaged by a surrounding canopy of stars. If (horrors) something happened to one of our inner planets, causing it to fall into the sun, a pea-sized pebble would surely be digested instantly, without so much as a burp.

Picture the precision slow-motion balance of gravity and acceleration that permit this particular pea-sized pebble of ours to orbit our beachball, without getting lost, once every 365-1/4 days.

Speaking of digestion, or indigestion, most astronomers think that our Sun will go “Red Giant” in about 5 billion years, when it runs out of hydrogen and bloats up in size, reaching out to about Earth’s current orbit. Our beachball, if you will, would then expand out some 180 times, until it fills two parking lots. Scary stuff. You might have to walk a quarter of a mile out to Jupiter to avoid a fast and permanent roasting.

But, not so fast! Remember that it takes about 6 minutes for sunlight to reach the real Earth from the real Sun. Within our scale model, the speed of light, about 186,284 miles per second, scales down to about 40 feet a minute. That’s a very leisurely stroll. If you walk faster than that, you’re cheating the laws of physics!

The Earth-Sun portion of our spreadsheet uses linear scaling and cubic mass and volume calculations, so some of the numeric results may not be adjusted correctly for scientific accuracy. They should certainly be “ballpark” enough to give us a better understanding of the relative mass, size and distance of the Sun, Earth and its moon. It’s a big neighborhood, with lots of empty space in between the standard signposts.

## History of Pluto

The object formerly known as the planet Pluto was discovered on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering. This period in astronomy was one of intense planet hunting, and Pickering was a prolific planet predictor.

In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed “Planet X.” By 1909, Lowell and Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted the search until his death in 1916, to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but they were not recognized for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909.

The search for Planet X did not resume until 1929, when the job was handed to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory. Tombaugh’s task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.

The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world the name Pluto was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England. Venetia was interested in classical mythology as well as astronomy, and considered the name for the god of the underworld appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford’s Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in the United States. Pluto officially became Pluto on March 24, 1930. The name was announced on May 1, 1930, and Venetia received five pounds (£5) as a reward.

A pair of small moons that NASA’s Hubble Space Telescope discovered orbiting Pluto now have official names: Nix and Hydra. NASA.

Published: 11/19/2019. Author: Science Reference Section, Library of Congress