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I'm trying to create a widget that shows the 'time' on different planets. It will show how far through the day/night cycle (as a percentage) a point on the planet is. It has been easy to scale down the cycle from 24 hr to eg ~10hr day of Jupiter however I am struggling to work out how I can 'set' the time.
For Mars this has been straight forward, using the comparison of MTC and UTC of when the Curiosity rover landed but I can't find similar data for other planets to anchor time from earth to the other planets.
Is there a data base for this kind of information?
NASA's Navigation Ancillary Facility (NAIF) publishes planetary constants kernels (PCK) which are basically text files containing pole orientations for the largest known bodies.
PCKs includes the parametric orientation of their prime meridian in agremeent with IAU standards (meaning datum is J2000.0).
The latest PCK dates from 2011 and is actively used by most operational interplanetary flight projects across the world.
Using the NAIF SPICE library (available in Fortran, C, Matlab and IDL) you can load this kernel and read the orientation of the prime meridian for your body and date of interest.
Time offset between bodies can be reconstructed as angular offset between meridians. But you may find some time-related functions within SPICE which can make your task easier.
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Not Wrong, Just Different
Yes and no. It is true that the meridian that runs through the observatory has lost its status as the world's sole reference point for longitude. Navigation systems such as the GPS now use the IERS Reference Meridian (IRM), which runs about 334 feet (102 meters) east of the observatory.
However, although the world is now using an updated version, the location of the prime meridian is not wrong as such&mdashit's just a different kind of meridian. It is defined by the location of the telescope, which was originally used to measure the passage of certain stars to feed data into an astronomical coordinate system, which, in its day, served as the basis for global navigation and timekeeping. Since the location of this original prime meridian is defined by the location of the telescope, it cannot be wrong: it is always where the telescope is.
What is the Prime Meridian - and why is it in Greenwich?
The Royal Observatory in Greenwich is where east meets west at Longitude 0°.
What is a meridian?
A meridian is a north-south line, selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it's possible to build up an accurate map of the sky.
Why does the Prime Meridian run through Greenwich?
There were two main reasons for the choice. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.
The decision was based on the argument that by naming Greenwich as Longitude 0º, it would be advantageous to the largest number of people. Therefore the Prime Meridian at Greenwich became the centre of world time.
Eastern and Western hemispheres
The line in Greenwich represents the historic Prime Meridian of the World - Longitude 0º. Every place on Earth was measured in terms of its distance east or west from this line. The line itself divided the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres. If you stand with one foot on one side and the other on the left, you are perfectly in the middle of east and west, according to the prime meridian line.
What is Greenwich Mean Time (GMT)?
Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time, or GMT.
Before this, almost every town in the world kept its own local time. There were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be.
When the railway and communications networks expanded in the 1850s and 1860s, there needed to be an international time standard. Greenwich was chosen as the centre for world time.
Where is the Prime Meridian?
In 1884 the Prime Meridian was defined by the position of the large 'Transit Circle' telescope in the Observatory’s Meridian Observatory. The transit circle was built by the 7th Astronomer Royal, Sir George Biddell Airy, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world.
As the Earth’s crust is moving very slightly all the time the exact position of the Prime Meridian is now moving very slightly too, but the original reference for the prime meridian of the world remains the Airy Transit Circle in the Royal Observatory, even if the exact location of the line may move to either side of Airy’s meridian.
What is the latitude of the Royal Observatory?
Where longitude is the distance east or west of the Prime Meridian line, latitude is measured by the distance north or south of the equator. Latitude and longitude are divided into degrees (°), minutes (′) and seconds (″), with sixty minutes in a degree and sixty seconds in a minute. The Royal Observatory lies at Longitude 0° by the original definition of Airy's Transit Circle, and at Latitude is 51° 28' 38'' N.
Where do the Prime Meridian and the Equator meet?
The intersection between these two invisble lines is in the middle of the Atlantic Ocean. It's a bit harder to get a selfie standing on this spot!
Who decided that the Prime Meridian should be in Greenwich?
The Greenwich Meridian was chosen as the Prime Meridian of the World in 1884. Forty-one delegates from 25 nations met in Washington DC for the International Meridian Conference. By the end of the conference, Greenwich had won the prize of Longitude 0º by a vote of 22 to 1 against (San Domingo), with 2 abstentions (France and Brazil).
Has the Meridian Line moved?
Between 1984 and 1988 an entirely new set of coordinate systems were adopted based on satellite data and other measurements and required a prime meridian that defined a plane passing through the centre of the Earth.
The true prime meridian of the world, as agreed by every nation on the planet in 1984, is the IERS Reference Meridian, which is also known as the International Reference Meridian or IRM.
The IRM is the only meridian that may now be described as the prime meridian of the world, as it defines 0 ° longitude by international agreement. The IRM passes 102.5 metres to the east of the historic Prime Meridian of the World at the latitude of the Airy Transit Circle here. The entire Observatory and the historic Prime Meridian now lie to the west of the true prime meridian.
Imagine floating in a lake for 24 hours a day—that's what it is like to be under Neptune. The energies are soft, spacey, and very fuzzy. Romanticism, art, music—all sorts of Neptunian activities—are front and center in the person’s life. The hard part of living under this line is not feeling clear about where you are in life. Illusions can take over and the individual might have a difficult time focusing on mundane tasks.
There is also be a tendency to try and escape life’s hard roads here—any sort of addictive behavior might be exacerbated.
PS1 The Great Design – Introduction to the Pyramid Matrix
“THE CODE” OF CARL MUNCK AND ANCIENT GEMATRIAN NUMBERS Hard Evidence of a Grand Design to Creation
The great mysteries of life are quite elusive. We do not have the “hard facts” needed to feel sure that our theories about the mysteries are true. Sometimes we feel sure, but convincing others is not so easy. Alas, they want “facts,” and we cannot produce them. Well, times are changing. This is the start of a series of articles that will present many “facts” concerning some major mysteries of our world. These “facts” will show evidence that:
- The ancient sites around the world are very precisely positioned on a global coordinate system in relation to the position of the Great Pyramid at Giza.
- The positions of the sites are given in the geometry of their construction.
- A very ancient system of numbers was used in the system, which we will call “Gematria.”
- “Gematrian” numbers are found in ancient myths and religions, including the Bible.
- Gematrian numbers were used in systems of weights and measures by ancient peoples, including the Greeks, the Egyptians, the Persians, the Babylonians and the Romans.
- The ancient Mayans used Gematrian numbers in their very accurate timekeeping.
- The Code system uses mathematical constants, such as pi and the radian.
- The system also uses conventions that are still in use, such as the 360 degree circle, 60 minute degree, 60 second minute, the base-ten numbering system, the 12-inch foot, and the 5280-foot mile.
- The Nazca Line ground markings “locate themselves” on The Code Matrix system.
- Crop circle formations suggest the same ancient numbers by way of their positionsand measurements.
- The very ancient “Monuments on Mars,” including “The Face on Mars,” were positioned in exact locations, just as the ancient sites on Earth.
Chichen Itza Pyramid
Latitude of the pyramid of Quetzalcoatl:
20° 40′ 58.44″ N
20 x 40 x 58.44 = 46752
This pyramid is a precise calendar (it has 91 steps on each of 4 sides plus platform on top: 4×91 +1 = 365).
The calendar connection is also confirmed by pyramid’s orientation marking equinoxes and solstices.
The pyramid has 4 sides with 4 staircases dividing each side into 2 sections (in total 8 sections).
Using these numbers and accurate value for 1 year equal 365.25, the pyramid “number” will perfectly relate its latitude:
365.25 x 4 x 4 x 8 = 46752
Note: 46752 = 365.25 x 128 (perhaps there is a better fit for the 128?)
Now let’s look at the coordinates of the Great Pyramid
N 29° 58’ 45.031” latitude and
E 31° 07’ 57.02” longitude ( or 0 o before the Prime Meridian was moved to Greenwich)
Let’s multiply latitude numbers:
29x58x45.031 = 75,742
First, this pyramid has 4 sides, its perfect slope angle is 51.8428° and there are 365.25 days in a year:
4 x 51.8428 x 365.25 = 75,742
Second, if we divide this number by 12, we get 6,312 this number very close to Earth Radius (mean 6,371 km).
Third, 12x60x60 = 43200 could be the scale of the GP.
Height of the Great Pyramid = 0.14664944 km,when multiplied by 43,200 gives 6335.25 km which is very close to value of the Earth’s Radius.
Another close coincidence:
The Speed Of Light, in a vacuum, is 299,792,458 m/s.
The King’s Chamber in the Khufu Pyramid is at Metric Geo Coordinates 29.9792458 N, 31.134197222 E.
Now, compare the North Latitude to the Speed Of Light.
Except for the fact it was converted to perform as a coordinate, the nine digits are identical.
The Great Design
Moon Radius is 1,738 km (1080 miles) and Earth Radius is 6,373 km (3960 miles).
Moon Radius/Earth Radius = 3/11 = 0.272727 which is the single design principle of the Great Pyramid.
It results in pyramid’s proportions (height to base ratio) 14/22 (7/11) – see images below.
The latitude of the Great Pyramid is 30 deg N
Astro-Geodetic Latitude of the Great Pyramid
The significance of the geodetic location of the Great Pyramid cannot be overstated, for it verifies that the shape and size of the Earth was known to the Ancients.
Professor Smyth’s determination of the Astro-Geodetic Latitude of the Great Pyramid in 1877 was:
29 Degrees 58 Minutes and 51 Seconds North (29.98083 Degrees)
His sighting was taken from on top of the Great Pyramid in order to reduce the effect of gravitational distortion of his plumb-line due to the mass of the Pyramid. This Astro-Geodetic Latitude was re-confirmed by Andre Pochan in 1978.
The present day geodetic location as given by GPS is:
29 Degrees 58 Minutes and 49 Seconds North (29.98027 Degrees)
A distinction must be made between the methods employed to derive the Geodetic locations. Obviously, taking sightings of stars would have been the means employed by the ancients to determine latitude as they would not have had satellite technology. As a comparison of accuracy, it is interesting to note that Airy’s Eye of the Greenwich Observatory in England (established as the Prime Meridian of the World in 1884) has a posted Latitude of 51 Degrees, 28 Minutes, 38 Seconds North which would be its Astro-Geodetic Latitude. The GPS coordinates of the Greenwich Observatory are 51 Degrees, 28 Minutes and 40 Seconds North. The reason for this variance is in part that the local gravity at any location of the Earth and hence a plumb-line, is influenced by the density of the Earth’s crust, which varies considerably from place to place. Particular variations occur in the proximity of mountain ranges and high iron concentrations in the crust strata. It is also to be given consideration that the exact position of the North Pole relative to the Earth experiences what is called “Polar Motion” due to changes in the center of Mass ie: changes in Ocean Currents.
This can effect Latitude readings by as much as 1.5 arc-seconds or 20 meters on the Earth’s surface. For all intents and purposes it is therefore to be given consideration that the Astro-Geodetic location of the Great Pyramid be taken as the location that the ancients would have had the means to measure. Any calculations for the Earth’s geometry giving consideration to the Pyramid’s location would have been contingent on the Astro-Geodetic determinations.
The Passage into the Queen’s Chamber provides a measurement which has relevance to the Pyramid’s location on Earth. If one follows the floor line from the major step back to the Grand Gallery and continuing a straight line through the minor step and proceeding down the slight incline to where the floor intersects the North wall of the Grand Gallery you will find this distance to be 1308.3 B”. Multiply this length by the Queen’s Chamber value (2400 x 1308.3) and one arrives at a product of 3,139,920 B” or 7,975,389.9 cm which equates to 79.7539 km. Subtract this value from the Reference Circumference (40,121.434 – 79.7539) and one arrives at a value of 40,041.680 km of which the radius would be 6372.831 km. It is of considerable interest to know that the radius of the Earth at the location of the Great Pyramid determined as at 29.98083 degrees North Geodetic Latitude is 6372.8306 km. It would appear that the location of the Pyramid was known to the Ancients to a level of accuracy comparable to that which was known in the late 1800’s. The radius of the Earth at 29.98083 degrees North was known. The difference in radius is only 0.7 meters.
The choice of 29.98083 degrees North Latitude appears at first glance to be either an attempt to fix its location at 30 degrees North, with a considerable inaccuracy, or that it was just simply a convenient location. Neither of these contentions appear to be the case when one gives careful consideration to the following mathematics which must have been considered in the choice of location of the Great Pyramid by the Ancients.
In order to understand the relevance of the location one must first calculate the Meridional Radius of Curvature for 29.98083 Degrees North Latitude, which is 6351.3585 km.
Finding a balance &mdash and losing it
The equilibrium state of a set of molecules, for instance in an alien atmosphere, is the ratios of substances they will eventually settle into, given a constant set of conditions and no outside influence. It's what one will generally expect to see when looking at the planet from afar.
"A planetary atmosphere has its own chemistry, but figuring out what the equilibrium state [is] (and therefore the relative abundances of molecules) is a big computational challenge because of all the reaction pathways and individual equilibria and the cross-talk of everything," Caleb Scharf, an exoplanet researcher at New York University, told Space.com in an email.
The quick calculations offered for particular combinations of molecules in the gas phase, as well as the authors' other work simplifying exoplanet climates, "is indeed quite a big deal," he added &mdash "It's a clever formulation that will speed up and improve the quest to pin down what's in planetary atmospheres" for a wide range of hotter atmospheric temperatures, he said.
Heng's group isn't the first to create simplified equations to describe an atmospheric makeup, but their calculations consider "the abundances of some of the most important molecules in exoplanet atmospheres, which can then easily and quickly be evaluated in an atmosphere model," David Amundsen, a researcher at Columbia University and NASA Goddard Institute for Space Studies, told Space.com in an email. While detailed atmosphere models will have to take a lot of other factors into account, this can simplify one part of the calculations &mdash exactly how much one might expect of each gas in the atmosphere.
And if some planet doesn't show an expected balance, does that mean something more interesting is going on?
"That's the million-dollar question, isn't it?" Heng said. "Because the classical thinking behind finding life is to first detect oxygen, ozone and water, and try to figure out if they're out of chemical equilibrium." If they are, he said, you search for other physical or geological causes for the disparity.
"And after you've exhausted these possibilities, then you dare start thinking about life," he added.
Eratosthenes in the 3rd century BCE first proposed a system of latitude and longitude for a map of the world. His prime meridian (line of longitude) passed through Alexandria and Rhodes, while his parallels (lines of latitude) were not regularly spaced, but passed through known locations, often at the expense of being straight lines.  By the 2nd century BCE Hipparchus was using a systematic coordinate system, based on dividing the circle into 360°, to uniquely specify places on Earth.  : 31 So longitudes could be expressed as degrees east or west of the primary meridian, as we do today (though the primary meridian is different). He also proposed a method of determining longitude by comparing the local time of a lunar eclipse at two different places, to obtain the difference in longitude between them.  : 11 This method was not very accurate, given the limitations of the available clocks, and it was seldom done – possibly only once, using the Arbela eclipse of 330 BCE.  But the method is sound, and this is the first recognition that longitude can be determined by accurate knowledge of time.
Ptolemy, in the 2nd century CE, developed these ideas and geographic data into a mapping system. Until then, all maps had used a rectangular grid with latitude and longitude as straight lines intersecting at right angles.  : 543  : 90 For large area this leads to unacceptable distortion, and for his map of the inhabited world, Ptolemy used projections (to use the modern term) with curved parallels that reduced the distortion. No maps (or manuscripts of his work) exist that are older than the 13th century, but in his Geography he gave detailed instructions and latitude and longitude coordinates for hundreds of locations that are sufficient to re-create the maps. While Ptolemy's system is well-founded, the actual data used are of very variable quality, leading to many inaccuracies and distortions.   : 551–553  The most important of these is a systematic over-estimation of differences in longitude. Thus from Ptolemy's tables, the difference in Longitude between Gibraltar and Sidon is 59° 40', compared to the modern value of 40° 23', about 48% too high. Luccio (2013) has analysed these discrepancies, and concludes that much of the error arises from Ptolemy's use of a much smaller estimate of the size of the earth than that given by Eratosthenes – 500 stadia to the degree rather than 700 (though Eratosthenes would not have used degrees). Given the difficulties of astronomical measures of longitude in classical times, most if not all of Ptolemy's values would have been obtained from distance measures and converted to longitude using the 500 value. Eratosthenes' result is closer to the true value than Ptolemy's. 
Ancient Hindu astronomers were aware of the method of determining longitude from lunar eclipses, assuming a spherical earth. The method is described in the Sûrya Siddhânta, a Sanskrit treatise on Indian astronomy thought to date from the late 4th century or early 5th century CE.  Longitudes were referred to a prime meridian passing through Avantī, the modern Ujjain. Positions relative to this meridian were expressed in terms of length or time differences, but not in degrees, which were not used in India at this time. It is not clear whether this method was actually used in practice.
Islamic scholars knew the work of Ptolemy from at least the 9th century CE, when the first translation of his Geography into Arabic was made. He was held in high regard, although his errors were known.  One of their developments was the construction of tables of geographical locations, with latitudes and longitudes, that added to the material provided by Ptolemy, and in some cases improved on it.  In most cases, the methods used to determine longitudes are not given, but there are a few accounts which give details. Simultaneous observations of two lunar eclipses at two locations were recorded by al-Battānī in 901, comparing Antakya with Raqqa. This allowed the difference in longitude between the two cities to be determined with an error of less than 1°. This is considered to be the best that can be achieved with the methods then available – observation of the eclipse with the naked eye, and determination of local time using an astrolabe to measure the altitude of a suitable "clock star".   Al-Bīrūnī, early in the 11th century CE, also used eclipse data, but developed an alternative method involving an early form of triangulation. For two locations differing in both longitude and latitude, if the latitudes and the distance between them are known, as well as the size of the earth, it is possible to calculate the difference in longitude. With this method, al-Bīrūnī estimated the longitude difference between Baghdad and Ghazni using distance estimates from travellers over two different routes (and with a somewhat arbitrary adjustment for the crookedness of the roads). His result for the longitude difference between the two cities differs by about 1° from the modern value.  Mercier (1992) notes that this is a substantial improvement over Ptolemy, and that a comparable further improvement in accuracy would not occur until the 17th century in Europe.  : 188
While knowledge of Ptolemy (and more generally of Greek science and philosophy) was growing in the Islamic world, it was declining in Europe. John Kirtland Wright's (1925) summary is bleak: "We may pass over the mathematical geography of the Christian period [in Europe] before 1100 no discoveries were made, nor were there any attempts to apply the results of older discoveries. . Ptolemy was forgotten and the labors of the Arabs in this field were as yet unknown".  : 65 Not all was lost or forgotten Bede in his De naturum rerum affirms the sphericity of the earth. But his arguments are those of Aristotle, taken from Pliny. Bede adds nothing original.   There is more of note in the later medieval period. Wright (1923) cites a description by Walcher of Malvern of a lunar eclipse in Italy (October 19, 1094), which occurred shortly before dawn. On his return to England, he compared notes with other monks to establish the time of their observation, which was before midnight. The comparison was too casual to allow a measurement of longitude differences, but the account shows that the principle was still understood.  : 81 In the 12th century, astronomical tables were prepared for a number of European cities, based on the work of al-Zarqālī in Toledo. These had to be adapted to the meridian of each city, and it is recorded that the lunar eclipse of September 12, 1178 was used to establish the longitude differences between Toledo, Marseilles, and Hereford.  : 85 The Hereford tables also added a list of over 70 locations, many in the Islamic world, with their longitudes and latitudes. These represent a great improvement on the similar tabulations of Ptolemy. For example, the longitudes of Ceuta and Tyre are given as 8° and 57° (east of the meridian of the Canary Islands), a difference of 49°, compared to the modern value of 40.5°, an overestimate of less than 20%.  : 87-88 In general, the later medieval period is marked by an increase in interest in geography, and of a willingness to make observations, stimulated both by an increase in travel (including pilgrimage and the Crusades) and the availability of Islamic sources from contact with Spain and North Africa   At the end of the medieval period, Ptolemy's work became directly available with the translations made in Florence at the end of the 14th- and beginning of the 15th-centuries. 
The 15th and 16th centuries were the time of Portuguese and Spanish voyages of discovery and conquest. In particular, the arrival of Europeans in the New World led to questions of where they actually were. Christopher Columbus made two attempts to use lunar eclipses to discover his longitude. The first was on Saona Island, now in the Dominican Republic, during his second voyage. He wrote: "In the year 1494, when I was in Saona Island, which stands at the eastern tip of Española island (i.e. Hispaniola), there was a lunar eclipse on September the 14th, and we noticed that there was a difference of more than five hours and a half between there [Saona] and Cape S.Vincente, in Portugal".  He was unable to compare his observations with ones in Europe, and it is assumed that he used astronomical tables for reference. The second was on the north coast of Jamaica on 29 February 1504 (during his fourth voyage). His determinations of latitude showed large errors of 13 and 38° W respectively.  Randles (1985) documents longitude measurement by the Portuguese and Spanish between 1514 and 1627 both in the Americas and Asia. Errors ranged from 2-25°. 
In 1608 a patent was submitted to the government in the Netherlands for a refracting telescope. The idea was picked up by, among others Galileo who made his first telescope the following year, and began his series of astronomical discoveries that included the satellites of Jupiter, the phases of Venus, and the resolution of the Milky Way into individual stars. Over the next half century, improvements in optics and the use of calibrated mountings, optical grids, and micrometers to adjust positions transformed the telescope from an observation device to an accurate measurement tool.     It also greatly increased the range of events that could be observed to determine longitude.
The second important technical development for longitude determination was the pendulum clock, patented by Christiaan Huygens in 1657.  This gave an increase in accuracy of about 30-fold over previous mechanical clocks – the best pendulum clocks were accurate to about 10 seconds per day.  From the start, Huygens intended his clocks to be used for determination of longitude at sea.   However, pendulum clocks did not tolerate the motion of a ship sufficiently well, and after a series of trials it was concluded that other approaches would be needed. The future of pendulum clocks would be on land. Together with telescopic instruments, they would revolutionise observational astronomy and cartography in the coming years.  Huygens was also the first to use a balance spring as oscillator in a working clock, and this allowed accurate portable timepieces to be made. But it was not until the work of John Harrison that such clocks became accurate enough to be used as marine chronometers. 
The development of the telescope and accurate clocks increased the range of methods that could be used to determine longitude. With one exception (magnetic declination) they all depend on a common principle, which was to determine an absolute time from an event or measurement and to compare the corresponding local time at two different locations. (Absolute here refers to a time that is the same for an observer anywhere on earth.) Each hour of difference of local time corresponds to a 15 degrees change of longitude (360 degrees divided by 24 hours).
Local noon is defined as the time at which the sun is at the highest point in the sky. This is hard to determine directly, as the apparent motion of the sun is nearly horizontal at noon. The usual approach was to take the mid-point between two times at which the sun was at the same altitude. With an unobstructed horizon, the mid-point between sunrise and sunset could be used.  At night local time could be obtained from the apparent rotation of the stars around the celestial pole, either measuring the altitude of a suitable star with a sextant, or the transit of a star across the meridian using a transit instrument. 
To determine the measure of absolute time, lunar eclipses continued to be used. Other proposed methods included:
Lunar distances Edit
This is the earliest proposal having been first suggested in a letter by Amerigo Vespucci referring to observations he made in 1499.   The method was published by Johannes Werner in 1514,  and discussed in detail by Petrus Apianus in 1524.  The method depends on the motion of the moon relative to the "fixed" stars, which completes a 360° circuit in 27.3 days on average (a lunar month), giving an observed movement of just over 0.5°/hour. Thus an accurate measurement of the angle is required, since 2 minute of arc (1/30°) difference in the angle between the moon and the selected star corresponds to a 1° difference in the longitude – 60 nautical miles at the equator.  The method also required accurate tables, which were complex to construct, since they had to take into account parallax and the various sources of irregularity in the orbit of the moon. Neither measuring instruments nor astronomical tables were accurate enough in the early 16th century. Vespucci's attempt to use the method placed him at 82° West of Cadiz, when he was actually less than 40° West of Cadiz, on the north coast of Brazil. 
Satellites of Jupiter Edit
In 1612, having determined the orbital periods of Jupiter's four brightest satellites (Io, Europa, Ganymede and Callisto), Galileo proposed that with sufficiently accurate knowledge of their orbits one could use their positions as a universal clock, which would make possible the determination of longitude. He worked on this problem from time to time during the remainder of his life.
The method required a telescope, as the moons are not visible to the naked eye. For use in marine navigation, Galileo proposed the celatone, a device in the form of a helmet with a telescope mounted so as to accommodate the motion of the observer on the ship.  This was later replaced with the idea of a pair of nested hemispheric shells separated by a bath of oil. This would provide a platform that would allow the observer to remain stationary as the ship rolled beneath him, in the manner of a gimballed platform. To provide for the determination of time from the observed moons' positions, a Jovilabe was offered this was an analogue computer that calculated time from the positions and that got its name from its similarities to an astrolabe.  The practical problems were severe and the method was never used at sea.
On land, this method proved useful and accurate. An early example was the measurement of the longitude of the site of Tycho Brahe's former observatory on the Island of Hven. Jean Picard on Hven and Cassini in Paris made observations during 1671 and 1672, and obtained a value of 42 minutes 10 seconds (time) east of Paris, corresponding to 10° 32' 30", about 12 minute of arc (1/5°) higher than the modern value. 
Appulses and occultations Edit
Two proposed methods depend on the relative motions of the moon and a star or planet. An appulse is the least apparent distance between the two objects, an occultation occurs when the star or planet passes behind the moon – essentially a type of eclipse. The times of either of these events can be used as the measure of absolute time in the same way as with a lunar eclipse. Edmond Halley described the use of this method to determine the longitude of Balasore in India, using observations of the star Aldebaran (the "Bull's Eye", being the brightest star in the constellation Taurus) in 1680, with an error of just over half a degree.  He published a more detailed account of the method in 1717.  A longitude determination using the occultation of a planet, Jupiter, was described by James Pound in 1714. 
The first to suggest travelling with a clock to determine longitude, in 1530, was Gemma Frisius, a physician, mathematician, cartographer, philosopher, and instrument maker from the Netherlands. The clock would be set to the local time of a starting point whose longitude was known, and the longitude of any other place could be determined by comparing its local time with the clock time.   : 259 While the method is perfectly sound, and was partly stimulated by recent improvements in the accuracy of mechanical clocks, it still requires far more accurate time-keeping than was available in Frisius's day. The term chronometer was not used until the following century,  and it would be over two centuries before this became the standard method for determining longitude at sea. 
Magnetic declination Edit
This method is based on the observation that a compass needle does not in general point exactly north. The angle between true north and the direction of the compass needle (magnetic north) is called the magnetic declination or variation, and its value varies from place to place. Several writers proposed that the size of magnetic declination could be used to determine longitude. Mercator suggested that the magnetic north pole was an island in the longitude of the Azores, where magnetic declination was, at that time, close to zero. These ideas were supported by Michiel Coignet in his Nautical Instruction. 
Halley made extensive studies of magnetic variation during his voyages on the pink Paramour. He published the first chart showing isogonic lines - lines of equal magnetic declination - in 1701.  One of the purposes of the chart was to aid in determining longitude, but the method was eventually to fail as changes in magnetic declination over time proved too large and too unreliable to provide a basis for navigation.
Measurements of longitude on land and sea complemented one another. As Edmond Halley pointed out in 1717, "But since it would be needless to enquire exactly what longitude a ship is in, when that of the port to which she is bound is still unknown it were to be wisht that the princes of the earth would cause such observations to be made, in the ports and on the principal head-lands of their dominions, each for his own, as might once for all settle truly the limits of the land and sea."  But determinations of longitude on land and sea did not develop in parallel.
On land, the period from the development of telescopes and pendulum clocks until the mid-18th century saw a steady increase in the number of places whose longitude had been determined with reasonable accuracy, often with errors of less than a degree, and nearly always within 2–3°. By the 1720s errors were consistently less than 1°. 
At sea during the same period, the situation was very different. Two problems proved intractable. The first was the need for immediate results. On land, an astronomer at, say, Cambridge Massachusetts could wait for the next lunar eclipse that would be visible both at Cambridge and in London set a pendulum clock to local time in the few days before the eclipse time the events of the eclipse send the details across the Atlantic and wait weeks or months to compare the results with a London colleague who had made similar observations calculate the longitude of Cambridge then send the results for publication, which might be a year or two after the eclipse.  And if either Cambridge or London had no visibility because of cloud, wait for the next eclipse. The marine navigator needed the results quickly. The second problem was the marine environment. Making accurate observations in an ocean swell is much harder than on land, and pendulum clocks do not work well in these conditions. Thus longitude at sea could only be estimated from dead reckoning (DR) – by using estimations of speed and course from a known starting position – at a time when longitude determination on land was becoming increasingly accurate.
In order to avoid problems with not knowing one's position accurately, navigators have, where possible, relied on taking advantage of their knowledge of latitude. They would sail to the latitude of their destination, turn toward their destination and follow a line of constant latitude. This was known as running down a westing (if westbound, easting otherwise).  This prevented a ship from taking the most direct route (a great circle) or a route with the most favourable winds and currents, extending the voyage by days or even weeks. This increased the likelihood of short rations,  which could lead to poor health or even death for members of the crew due to scurvy or starvation, with resultant risk to the ship.
A famous longitude error that had disastrous consequences occurred in April 1741. George Anson, commanding HMS Centurion, was rounding Cape Horn from east to west. Believing himself past the Cape, he headed north, only to find the land straight ahead. A particularly strong easterly current had put him well to the east of his dead-reckoning position, and he had to resume his westerly course for several days. When finally past the Horn, he headed north for the Juan Fernández Islands, to take on supplies, and to relieve his crew, many of whom were sick with scurvy. On reaching the latitude of Juan Fernández, he did not know whether the islands were to the east or west, and spent 10 days sailing first eastwards and then westwards before finally reaching the islands. During this time over half of the ship's company died of scurvy.  
In response to the problems of navigation, a number of European maritime powers offered prizes for a method to determine longitude at sea. Spain was the first, offering a reward for a solution in 1567, and this was increased to a permanent pension in 1598. Holland offered 30,000 florins in the early 17th century. Neither of these prizes produced a solution.  : 9
The second half of the 17th century saw the foundation of two observatories, one in Paris and the other in London. The Paris Observatory was the first, being founded as an offshoot of the French Académie des Sciences in 1667. The Observatory building, to the south of Paris, was completed in 1672.  Early astronomers included Jean Picard, Christiaan Huygens, and Dominique Cassini.  : 165–177 The Observatory was not set up for any specific project, but soon became involved in the survey of France that led (after many delays due to wars and unsympathetic ministries) to the Academy's first map of France in 1744. The survey used a combination of triangulation and astronomical observations, with the satellites of Jupiter used to determine longitude. By 1684, sufficient data had been obtained to show that previous maps of France had a major longitude error, showing the Atlantic coast too far to the west. In fact France was found to be substantially smaller than previously thought.  
The Royal Observatory, Greenwich, to the east of London, was set up a few years later in 1675, and was established explicitly to address the longitude problem.  John Flamsteed, the first Astronomer Royal was instructed to "apply himself with the utmost care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, so as to find out the so-much-desired longitude of places for the perfecting the art of navigation".  : 268  The initial work was in cataloguing stars and their position, and Flamsteed created a catalogue of 3,310 stars, which formed the basis for future work.  : 277
While Flamsteed's catalogue was important, it did not in itself provide a solution. In 1714, the British Parliament passed “An Act for providing a public Reward for such Person or Persons as shall discover the Longitude at Sea”, and set up a Board to administer the award. The rewards depended on the accuracy of the method: from £10,000 for an accuracy within one degree of latitude (60 nautical miles at the equator) to £20,000 for accuracy within one-half of a degree.  : 9
This prize in due course produced two workable solutions. The first was lunar distances, which required careful observation, accurate tables, and rather lengthy calculations. Tobias Mayer had produced tables based on his own observations of the moon, and submitted these to the Board in 1755. These observations were found to give the required accuracy, although the lengthy calculations required (up to four hours) were a barrier to routine use. Mayer's widow in due course received an award from the Board.  Nevil Maskelyne, the newly appointed Astronomer Royal who was on the Board of Longitude, started with Mayer's tables and after his own experiments at sea trying out the lunar distance method, proposed annual publication of pre-calculated lunar distance predictions in an official nautical almanac for the purpose of finding longitude at sea. Being very enthusiastic for the lunar distance method, Maskelyne and his team of computers worked feverishly through the year 1766, preparing tables for the new Nautical Almanac and Astronomical Ephemeris. Published first with data for the year 1767, it included daily tables of the positions of the Sun, Moon, and planets and other astronomical data, as well as tables of lunar distances giving the distance of the Moon from the Sun and nine stars suitable for lunar observations (ten stars for the first few years).    This publication later became the standard almanac for mariners worldwide. Since it was based on the Royal Observatory, it helped lead to the international adoption a century later of the Greenwich Meridian as an international standard.
The second method was the use of chronometer. Many, including Isaac Newton, were pessimistic that a clock of the required accuracy could ever be developed. A degree of longitude is equivalent to four minutes of time,  so the required accuracy is a few seconds a day. At that time, there were no clocks that could come close to maintaining such accurate time while being subjected to the conditions of a moving ship. John Harrison, a Yorkshire carpenter and clock-maker believed it could be done, and spent over three decades proving it.  : 14-27
Harrison built five chronometers, two of which were tested at sea. His first, H-1, was not tested under the conditions that were required by the Board of Longitude. Instead, the Admiralty required that it travel to Lisbon and back. It lost considerable time on the outward voyage but performed excellently on the return leg, which was not part of the official trial. The perfectionist in Harrison prevented him from sending it on the required trial to the West Indies (and in any case it was regarded as too large and impractical for service use). He instead embarked on the construction of H-2. This chronometer never went to sea, and was immediately followed by H-3. During construction of H-3, Harrison realised that the loss of time of the H-1 on the Lisbon outward voyage was due to the mechanism losing time every time the ship came about while tacking down the English Channel. Harrison produced H-4, with a completely different mechanism which did get its sea trial and satisfied all the requirements for the Longitude Prize. However, he was not awarded the prize by the Board and was forced to fight for his reward, finally receiving payment in 1773, after the intervention of parliament  : 26 .
The French were also very interested in the problem of Longitude, and the Academy examined proposals and also offered prize money, particularly after 1748.  : 160 Initially the assessors were dominated by the astronomer Pierre Bouguer who was opposed to the idea of chronometers, but after his death in 1758 both astronomical and mechanical approaches were considered. Two clock-makers dominated, Ferdinand Berthoud and Pierre Le Roy. Four sea trials took place between 1767 and 1772, evaluating lunar distances as well as a variety of time-keepers. Results for both approaches steadily improved as the trials proceeded, and both methods were deemed suitable for use in navigation.  : 163-174
Although both chronometers and lunar distances had been shown to be practicable methods for determining longitude, it was some while before either became widely used. In the early years, chronometers were very expensive, and the calculations required for lunar distances were still complex and time consuming, in spite of Maskelyne's work to simplify them. Both methods were initially used mainly in specialist scientific and surveying voyages. On the evidence of ships' logbooks and nautical manuals, lunar distances started to be used by ordinary navigators in the 1780s, and became common after 1790. 
While chronometers could deal with the conditions of a ship at sea, they could be vulnerable to the harsher conditions of land-based exploration and surveying, for example in the American North-West, and lunar distances were the main method used by surveyors such as David Thompson.  Between January and May 1793 he took 34 observations at Cumberland House, Saskatchewan, obtaining a mean value of 102° 12' W, about 2' (2.2 km) east of the modern value.  Each of the 34 observations would have required about 3 hours of calculation. These lunar distance calculations became substantially simpler in 1805, with the publication of tables using the Haversine method by Josef de Mendoza y Ríos. 
The advantage of using chronometers was that though astronomical observations were still needed to establish local time, the observations were simpler and less demanding of accuracy. Once local time had been established, and any necessary corrections made to the chronometer time, the calculation to obtain longitude was straightforward. The disadvantage of cost gradually became less as chronometers began to be made in quantity. The chronometers used were not those of Harrison. Other makers in particular Thomas Earnshaw, who developed the spring detent escapement,  simplified chronometer design and production. From 1800 to 1850, as chronometers became more affordable and reliable, they increasingly displaced the lunar distance method.
Chronometers needed to be checked and reset at intervals. On short voyages between places of known longitude this was not a problem. For longer journeys, particularly of survey and exploration, astronomical methods continued to be important. An example of the way chronometers and lunars complemented one another in surveying work is Matthew Flinders' circumnavigation of Australia in 1801-3. Surveying the south coast, Flinders started at King George Sound, a known location from George Vancouver's earlier survey. He proceeded along the south coast, using chronometers to determine longitude of the features along the way. Arriving at the bay he named Port Lincoln, he set up a shore observatory, and determined the longitude from thirty sets of lunar distances. He then determined the chronometer error, and recalculated all the longitudes of the intervening locations. 
Ships often carried more than one chronometer. Two provided dual modular redundancy, allowing a backup if one should cease to work, but not allowing any error correction if the two displayed a different time, since in case of contradiction between the two chronometers, it would be impossible to know which one was wrong (the error detection obtained would be the same of having only one chronometer and checking it periodically: every day at noon against dead reckoning). Three chronometers provided triple modular redundancy, allowing error correction if one of the three was wrong, so the pilot would take the average of the two with closer readings (average precision vote). There is an old adage to this effect, stating: "Never go to sea with two chronometers take one or three."  Some vessels carried more than three chronometers – for example, HMS Beagle carried 22 chronometers. 
By 1850, the vast majority of ocean-going navigators worldwide had ceased using the method of lunar distances. Nonetheless, expert navigators continued to learn lunars as late as 1905, though for most this was a textbook exercise since they were a requirement for certain licenses. Littlehales noted in 1909: "The lunar-distance tables were omitted from the Connaissance des Temps for the year 1905, after having retained their place in the French official ephemeris for 131 years and from the British Nautical Almanac for 1907, after having been presented annually since the year 1767, when Maskelyne's tables were published." 
Surveying on land continued to use a mixture of triangulation and astronomical methods, to which was added the use of chronometers once they became readily available. An early use of chronometers in land surveying was reported by Simeon Borden in his survey of Massachusetts in 1846. Having checked Nathaniel Bowditch's value for the longitude of the State House in Boston he determined the longitude of the First Congregational Church at Pittsfield, transporting 38 chronometers on 13 excursions between the two locations.  Chronometers were also transported much longer distances. For example the US Coast Survey organised expeditions in 1849 and 1855 in which a total of over 200 chronometers were shipped between Liverpool and Boston, not for navigation, but to obtain a more accurate determination of the longitude of the Observatory at Cambridge, Massachusetts, and thus to anchor the US Survey to the Greenwich meridian.  : 5
The first working telegraphs were established in Britain by Wheatstone and Cooke in 1839, and in the USA by Morse in 1844. The idea of using the telegraph to transmit a time signal for longitude determination was suggested by François Arago to Morse in 1837,  and the first test of this idea was made by Capt. Wilkes of the U.S. Navy in 1844, over Morse's line between Washington and Baltimore. Two chronometers were synchronized, and taken to the two telegraph offices to conduct the test and check that time was accurately transmitted. 
The method was soon in practical use for longitude determination, in particular by the U.S. Coast Survey, and over longer and longer distances as the telegraph network spread across North America. Many technical challenges were dealt with. Initially operators sent signals manually and listened for clicks on the line and compared them with clock ticks, estimating fractions of a second. Circuit breaking clocks and pen recorders were introduced in 1849 to automate these process, leading to great improvements in both accuracy and productivity.  : 318–330  : 98–107
A big expansion to the "telegraphic net of longitude" was due to the successful completion of the transatlantic telegraph cable between S.W. Ireland and Nova Scotia in 1866.  A cable from Brest in France to Duxbury Massachusetts was completed in 1870, and gave the opportunity to check results by a different route. In the interval, the land-based parts of the network had improved, including the elimination of repeaters. Comparisons of the difference between Greenwich and Cambridge Massachusetts showed differences between measurement of 0.01 second of time, with a probable error of ±0.04 seconds, equivalent to 45 feet.  : 175 Summing up the net in 1897, Charles Schott presented a table of the major locations throughout the United States whose locations had been determined by telegraphy, with the dates and pairings, and the probable error.   The net was expanded into the American North-West with telegraphic connection to Alaska and western Canada. Telegraphic links between Dawson City, Yukon, Fort Egbert, Alaska, and Seattle and Vancouver were used to provide a double determination of the position of the 141st meridian where it crossed the Yukon River, and thus provide a starting point for a survey of the border between the USA and Canada to north and south during 1906–1908  
The U.S. Navy expanded the web into the West Indies and Central and South America in four expeditions in the years 1874-90. One series of observations linked Key West, Florida with the West Indies and Panama City.  A second covered locations in Brazil and Argentina, and also linked to Greenwich via Lisbon.  The third ran from Galveston, Texas, through Mexico and Central America, including Panama, and on to Peru and Chile, connecting to Argentina via Cordoba.  The fourth added locations in Mexico, Central America and the West Indies, and extended the chain to Curaçao and Venezuela. 
East of Greenwich, telegraphic determinations of longitude were made of locations in Egypt, including Suez, as part of the observations of the 1874 transit of Venus directed by Sir George Airy, the British Astronomer Royal.   Telegraphic observations made as part of the Great Trigonometrical Survey of India, including Madras, were linked to Aden and Suez in 1877.   In 1875, the longitude of Vladivostok in eastern Siberia was determined by telegraphic connection with Saint Petersburg. The US Navy used Suez, Madras and Vladivostok as the anchor-points for a chain of determinations made in 1881–1882, which extended through Japan, China, the Philippines, and Singapore. 
The telegraphic web circled the globe in 1902 with the connection of Australia and New Zealand to Canada via the All Red Line. This allowed a double determination of longitudes from east and west, which agreed within one second of arc (1/15 second of time). 
The telegraphic net of longitude was less important in Western Europe, which had already mostly been surveyed in detail using triangulation and astronomical observations. But the "American Method" was used in Europe, for example in a series of measurements to determine the longitude difference between the observatories of Greenwich and Paris with greater accuracy than previously available. 
Marconi was granted his patent for wireless telegraphy in 1897.  The potential for using wireless time signals for determining longitude was soon apparent. 
Wireless telegraphy was used to extend and refine the telegraphic web of longitude, giving potentially greater accuracy, and reaching locations that were not connected to the wired telegraph network. An early determination was that between Potsdam and The Brocken in Germany, a distance of about 100 miles, in 1906.  In 1911 the French determined the difference of longitude between Paris and Bizerte in Tunisia, a distance of 920 miles, and in 1913-14 a transatlantic determination was made between Paris and Washington. 
The first wireless time signals for the use of ships at sea started in 1907, from Halifax, Nova Scotia.  Time signals were transmitted from the Eiffel Tower in Paris starting in 1910.  These signals allowed navigators to check and adjust their chronometers on a frequent basis.   An international conference in 1912 allocated times for various wireless stations around the world to transmit their signals, allowing for near-worldwide coverage without interference between stations.  Wireless time-signals were also used by land-based observers in the field, in particular surveyors and explorers. 
Radio navigation systems came into general use after World War II. Several systems were developed including the Decca Navigator System, the US coastguard LORAN-C, the international Omega system, and the Soviet Alpha and CHAYKA. The systems all depended on transmissions from fixed navigational beacons. A ship-board receiver calculated the vessel's position from these transmissions.  These systems were the first to allow accurate navigation when astronomical observations could not be made because of poor visibility, and became the established method for commercial shipping until the introduction of satellite-based navigation systems in the early 1990s.
In the densest fog or darkness of night, without a compass or other instruments of orientation, or a timepiece, it will be possible to guide a vessel along the shortest or orthodromic path, to instantly read the latitude and longitude, the hour, the distance from any point, and the true speed and direction of movement. 
His prediction was fulfilled partially with radio navigation systems, and completely with modern computer systems based on GPS.
Your Questions About Gravitational Waves, Answered
Gizmodo readers asked a lot of great questions about yesterday’s big announcement on the discovery of gravitational waves. And Dr. Amber Stuver of the LIGO Livingston Observatory in Louisiana is here today with some answers.
Gizmodo Readers: There has been a huge amount of work put into the detection of a single gravitational wave to this point and it is a huge breakthrough. It sure seems this could open up a lot of new exciting possibilities in astronomy - but is this first detection “merely” a proof that the detection in itself is possible or will you already be able to gain further scientific advancements from this? What do you hope to do with this in the future? Will there be easier methods of detecting these waves in the future?
Stuver: This is indeed the first detection, which is a breakthrough, but the goal has always been to use gravitational waves to do new astronomy. Instead of looking into the universe seeing light, now we are able to feel the very small changes in gravity caused by some of the largest, most violent, and (in my opinion) most interesting things in the universe—including things that light will never be able to bring us information about.
We have been able to do this new kind of astronomy using the waves of this first detection. Using what we already know about general relativity, we can predict what gravitational waves from objects such as black holes or neutron stars look like. The signal we found matches what is predicted for a pair of black holes, one 36 times as massive as our Sun and the other 29 times, orbiting each other faster and faster as they get close together. Finally, they merge into one black hole. So, not only was this the first detection of gravitational waves, it was also the first direct observation of black holes since light cannot be used to observe them (only how the matter around them moves).
How can you be certain that outside effects (such as vibration) aren’t impacting the results?
Stuver: At LIGO, we record much more data related to our environment and equipment than data that can contain a gravitational wave signal. The reason for this is that we want to be as sure as possible that outside effects don’t trick us into thinking we’ve discovered a gravitational wave. If the ground was moving an abnormal amount at the time we think we see a gravitational wave signal, we will likely dismiss it as a detection candidate.
Another measure we take to not see something accidentally is that both LIGO detectors must see the same signal within the amount of time it would take a gravitational wave to travel between the two facilities. The maximum amount of time for this trip is about 10 milliseconds. To be considered a potential detection, we must see a signal with the same shape and almost the same time and the extra data we collect from our environment must be clean of abnormalities.
There are also many other tests a detection candidate must pass before we consider it to be a valid detection, but these are the basics.
Is there any practical way for us to generate gravity waves that could be detected by a device such as this? So that we could build a gravity radio or laser?
Stuver: What you suggest is exactly what Heinrich Hertz did in the late 1880s to detect electromagnetic waves in the form of radio waves. However, gravity is the weakest of the fundamental forces holding the universe together. Because of that, moving masses around in a lab or other facility will create gravitational waves but they will be too weak for even sensitive detectors like LIGO. To make the waves strong enough, we would need to spin a dumbbell at speeds so high that it would rip any known material apart in the process. The next place to look for large amounts of mass moving around extremely fast is the universe and that is why we build detectors that target these far away sources.
Will this confirmation change our future at all? Could we harness the power of these waves for space exploration for example? Would it be possible to communicate via these waves?
Stuver: Because of the amount of mass that needs to be moving with extreme acceleration to produce gravitational waves detectors like LIGO can detect, the only known mechanism for this is pairs of neutron stars or black holes orbiting just before they merge into one (there are other sources too, but this works for our discussion). The chances of there being an advanced civilization with the means to manipulate matter like this are incredibly small. Even if these civilizations did exist, there are much more efficient ways to communicate with us. Personally, I don’t think it would end well for us if we encountered a civilization that had the ability to use gravitational waves as a means of communication since they would also be able to destroy us handily.
Are gravity waves coherent? Can they be made to be coherent? Can they be focused? What would be the effect upon a massive object of being subjected to a focused beam of gravity? Could this effect be employed to improve particle accelerators?
Stuver: Certain kinds of gravitational waves can be coherent. Consider a neutron star that is nearly perfectly spherical. If it is spinning quickly, small deformities of less than an inch will produce gravitational waves of a very consistent frequency making them coherent. But it is very difficult to focus gravitational waves because the universe is transparent to them that is, gravitational waves pass through matter and come out unchanged. You would need to change the path of at least part of the gravitational waves in order to focus them. There may be an exotic form of gravitational lensing that could at least partially focus gravitational waves, but these would be difficult if not impossible to use for a purpose. If they could be focused, they are still so weak that I don’t know of a practical application they could have. But that is also what they said about lasers, which are just focused coherent light, so who knows?
What is the speed of a gravitational wave? Does it have mass? If it doesn’t have mass, is it possible for it to move faster than the speed of light?
Stuver: Gravitational waves are expected to travel at the speed of light. This is the speed that is implied by general relativity. However, experiments like LIGO will get to test this. It is possible that they could travel slower but very near the speed of light. If that is the case, then the theoretical particle associated with gravity (and what gravitational waves are made up of) called the graviton would have a mass. Since gravity acts between masses, this would add complications into the theory. The complications don’t make it impossible, just improbable. This is a great example of the use of Occam’s razor: the simplest explanation is usually the correct one.
How far away do you have to be from this kind of black hole merger to live to tell the tale?
Stuver: For the black hole binary we detected with gravitational waves, they produced a maximum change in the length of our 4 km (
2.5 mi) long arms [of] 1x10 -18 meters (that is 1/1000 the diameter of a proton). We also estimate that these black holes were 1.3 billion light-years away.
Now assume that we are 2 m (
6.5 ft) tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165 nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright). This is more than survivable.
Using this new sense to listen to the cosmos, what are some big areas on which scientists are focusing to find out more about the universe?
Stuver: The potential is really unknown, meaning that there will be many more areas than we have even though up so far. The more we learn about the universe, the better the questions we will be able to answer with gravitational waves. Just a few are below:
· What is the cause of gamma-ray bursts?
· How does matter behave in the extreme environment of a collapsing star?
· What were the first instants after the Big Bang like?
· How does matter in neutron stars behave?
But what I am really interested in is discovering gravitational waves we didn’t anticipate. Every time humans have observed the universe in a new way, we’ve always discovered something unexpected that revolutionized our understanding of the universe. I want to find those gravitational waves and find something we couldn’t have even imagined before.
Will this have any impact on the possibility of ever making a real warp drive?
Stuver: Since gravitational waves don’t have a significant interaction with matter, there really isn’t a way to use them to propel matter. Even if you could, a gravitational wave could only travel up to the speed of light. Using them as a means to power a warp drive to go faster than the speed of light isn’t possible. I wish it was though!
What are the implications now about anti-gravity devices?
Stuver: For an anti-gravity device we would need to turn the attractive gravitational force into a repulsive one. While a gravitational wave is a propagating change in gravity, this change never becomes repulsive (i.e. negative).
Why gravity is always attractive is because negative mass doesn’t seem to exist. After all, there are positive and negative electric charges, north and south magnetic poles, but only positive mass. Why? If there was negative mass, a ball made out of the stuff would fall up instead of down. It would be repulsed from the positive mass of the Earth.
What does this mean for the possibility of time-travel and teleportation? Could we conceivably find practical applications for this phenomenon beyond learning about the universe?
Stuver: Right now, the best ways to time travel (and only into the future) are to travel round-trip at nearly the speed of light (this is the twin-paradox of special relativity) or to move into an area with much higher gravity (this is like the general relativity time travel that was featured on Interstellar). Since a gravitational wave is a propagating change in gravity, there will be very small fluctuations in the speed of time, but since gravitational waves are inherently weak, the time fluctuations are as well. While I can’t think of a practical application towards time travel (or teleportation), I’ve learned to never say never (but don’t hold your breath either).
Do you anticipate a day when we stop confirming Einstein and start finding unexpected weirdness again? At least in cosmological physics terms it sometimes feels like we live in a world where Nostradamus wrote clearly and in English.
Stuver: Absolutely! Since gravity is the weakest of the forces, it is also the hardest to test. So far, every time Einstein’s relativity has been put to the test, it has accurately predicted the results of those experiments. Even the tests of general relativity we were able to do with the gravitational waves we have detected have confirmed general relativity. But I expect that we are going to be able to start testing such fine details of the theory (maybe with gravitational waves or other ways) that we will start seeing “funny” things, like having the results of an experiment be very close to what was expected, but not exactly. That won’t mean that relativity is wrong, it just may need to have some of its details refined.
Every time we answer one question about nature, it leads to more. Eventually we will have questions that will be “more” than general relativity can completely explain. That’s what makes being a scientist exciting.
Can you explain how this discovery relates to/affects the Unified Field Theory? Are we closer to confirming that or closer to debunking it?
Stuver: Right now, the results of the discovery we’ve made focus mainly on testing and confirming general relativity. Unified field theory seeks to develop a theory that can explain the physics of the very small (quantum mechanics) and the very large (general relativity). Right now, these two theories can be generalized that explain the scale of the world we live in, but not past that. Since this is focusing on the physics of the very large, there isn’t much that this discovery alone can do to advance us towards a unified theory. But as we learn more is it not out of the question. Right now, the field of gravitational wave physics is newly born. As we learn more, we will be able to possibly extend it to work toward a unified theory. But we must walk before we can run.
Now that we’re listening to gravity waves, what’s the most outrageous thing we might hear that could cause scientists to lose their collective shit? (1) Unnatural patterns/structure? (2) Gravity wave source from a region we were certain was empty? (3) Rick Astley - Never gonna give you up?
Stuver: As soon as I read your question, I immediately thought of the scene in Contact where the radio telescope picks up patterns of prime numbers. This is not anything that would be naturally occurring (at least that anyone has thought of yet). So the unnatural pattern/structure you suggest is something I think would be most likely.
I’m not sure that we are ever sure that a certain part of space is empty. After all, the black hole system we found was isolated and no light would ever come from this region, but we found gravitational waves there anyway.
Now music… I specialize in separating gravitational wave signals from the static-like noise we constantly measure from environmental influences. If I found music as a gravitational wave, especially music I’ve heard before, I would know that I was on the receiving end of a practical joke. But music that has never been heard on Earth before… That would rank up there with the prime number sequence from Contact.
Since the experiment detects the wave by a change in distance between two locations, is the amplitude greater in one direction than the other? IOW, do the readings imply that the universe is changing in size? And if so, does it confirm expansion, or something unexpected?
Stuver: We would need to observe many gravitational waves coming from many different directions in the universe before we could begin to answer that. In astronomy, this is creating a population model. How many different kinds of things are where? That’s the main question. Once we have many observations and start seeing uneven patterns, like there are many more of this kind of gravitational wave coming from a certain part of the universe and almost never anywhere else, that would be an extremely interesting result. Some patterns could confirm expansion (which we are already very sure of) or other phenomena that we haven’t thought of yet. But we need to see many more gravitational waves first.
I don’t have time to read the article, but I read the earlier post today, and it didn’t explain how, exactly, they knew the waves that were measured were due to these two supermassive blackholes. How can they know precisely what caused any of the waves they measure?
Stuver: The data analysis methods used a catalog of predicted gravitational wave signals to compare to our data. If there is a strong correlation to one of these predications, or templates, then we not only know this is a gravitational wave candidate, but we also know what system made it from what system was used to create the template.
Each different way of making a gravitational wave, black holes merging (like this discovery), stars orbiting each other, stars dying in explosions or creating black holes, all of these will have different shapes. When we detect a gravitational wave, we use these shapes as predicted by general relativity to determine what caused it.
How do we know that these waves originated from the collision of two black holes and not some other event? is it possible to have any idea where/when the event happened to any degree of accuracy?
Stuver: Once we know what system made the gravitational wave, we can predict how strong the gravitational wave was near where it was produced. Measuring its strength once it gets to Earth and comparing our measurement to the predicted strength at the source, we can calculate how far away the source is. Since gravitational waves travel at the speed of light, we also can calculate how long the gravitational waves have been traveling to Earth.
For the black hole system we discovered, we measured a maximum change in the length of LIGO’s arms of 1/1000 the diameter of a proton. For this system, that places it about 1.3 billion light-years away. So, the gravitational wave discovered in September and announced yesterday have been ontheir way to us for 1.3 billion years. This was before animal life formed on Earth but after multicellular life formed.
During the announcement mention was made of other detectors to look at longer wave periods—some of the graphics shown imply that these would be space-based detectors. Can you give more details about these larger detectors—where we are in implementation, what additional challenges we face to create them, how long it will take to get them on line, what types of information they could yield versus LIGO?
Stuver: There is indeed a space-based detector in the works. It is called LISA (Laser InterferometerSpace Antenna) . Since it would be in space, it would be sensitive to low frequency gravitational waves that ground-based detectors won’t be due to the Earth’s natural vibrations. This is a huge technological challenge since these satellites need to be farther away from Earth than humans have ever gone. That means if something malfunctions, we can’t send astronauts up to fix it like we did the Hubble in the 1990’s. To test the needed technologies, a mission called LISA Pathfinder was launched in December 2015. So far, it has completed all of its benchmarks successfully but this mission isn’t over yet.
Can gravitational waves be translated into sound waves? And if so, what would it sound like?
Stuver: Absolutely. Of course, you can’t hear just a gravitational wave. But if you take the signal and put it through speakers, you can hear it.
What can we do with this information? Do other astronomical objects of sizable mass emit these waves? Can this be used to find planets or just black holes?
Stuver: It isn’t just the mass that matters when looking for gravitational waves (although more is better). It is also the acceleration that the object is undergoing. The black holes we discovered were orbiting around each other at about 60 percent the speed of light when they merged. That is why we could detect them as they were merging. But there are no more gravitational waves coming from them now that they have settled down into one mass with little motion.
So anything very massive moving around very quickly can possibly make gravitational waves we can detect.
Exoplanets are much less likely to have the mass or the acceleration to make detectable gravitational waves. (I’m not saying they aren’t making gravitational waves, just that they aren’t strong enough or the right frequency for us to detect). Even if an exoplanet were massive enough to possibly make detectable gravitational waves, the accelerations that it would need to undergo would likely tear the planet apart. This is especially an issue since the most massive planets tend to be gas giants.
With the establishment of the prime meridian and zero degrees longitude at Greenwich, the conference also established time zones. By establishing the prime meridian and zero degrees longitude in Greenwich, the world was then divided into 24 time zones (since the earth takes 24 hours to revolve on its axis) and thus each time zone was established every fifteen degrees of longitude, for a total of 360 degrees in a circle.
The establishment of the prime meridian in Greenwich in 1884 permanently established the system of latitude and longitude and time zones that we use to this day. Latitude and longitude are used in GPS and is the primary coordinate system for navigation on the planet.