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An ephemeris is a star chart and almanac. It’s a table of values that tells the exact position of stars, planets, and other celestial bodies at given times. This chart is used in astrology and astronomy. The first known ephemeris was created in 2000 BC in India. It is called the Vedic Period and is based off the Panchanga tables. As time progressed, the knowledge of these star charts migrated from the East into Western Europe primarily through the Muslim world.
The first European Ephemeris was created in the 12th century AD. It was based off a series Arab Ephemerides called the Zij. The Zij was a series of ephemerides that were constructed from the Vedic period and the observations of the Greek Philosopher Ptolemy. Thanks to the extent of the Islamic Empire through the following centuries this along with other ancient disciplines preserved by Islamic culture were eventually transmitted to Europe.
The First European Ephemeris was edited by Gerard of Cremona. He and a group of other European scholars translated Greek and Arabic astronomy text and other text on medicine and the sciences into Latin, the universal language of Europe at the time. This lead to the wide dissemination of this knowledge throughout western civilizations
Through succeeding centuries, the western Ephemeris was perfected as the observations of astronomers like Copernicus helped to create a more accurate picture of the movement of the planets eventually producing the Prutenic Tables. These tables would be further improved by the observation of Johannes Kepler who later produced his own improved ephemeris based on his Laws of Planetary Motion. This table is called the Rudolphine Tables in 1627.
In modern astronomy, due to new instruments that allow for more accurate observation of celestial bodies, ephemerides are more accurate than ever before allowing us to not only know the position of a star, planet, asteroid, or comet at any given time but do it centuries into the past or future. Despite these advances there are still some flaws. There are local phenomenon such as asteroids which we don’t have complete information about so the tables need to be updated every 20 years. Also, there is the fact that an accurate ephemeris can only be accurate for the location it was made so a single table might have to be calibrated several times to be accurate from other locations.
Ephemerides also play an important role in spaceflight. NASA uses these tables to help plot the flight of probes to other planets and through the rest of the solar system.
If you enjoyed this article there are several others on astronomy on Universe Today. There is an interesting article about the debate over the Christmas star that shows the role an ephemeris can play in learning about past stellar phenomenon. You should also check out this article about the Messenger Mission.
There are also some interesting articles on the web. This site has an interesting set of articles on the history astronomy. There is also link to a informative paper that talks specifically about the history of astronomy in India.
You can also find interesting episodes to listen to on Astronomy Cast. Try listening to episode 133 on Optical Astronomy.