What’s an ephemeris?

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An ephemeris is a table that provides information about the positions of celestial objects such as stars and planets on given dates in the past or future. It can also show the positions of artificial satellites. The term, the plural of which is ephemeris, comes from the Greek word for diary or diary. Since ancient times, humans have tracked the positions of stars and planets in the sky, for a variety of reasons related to religion, navigation, and more recently, space travel. A cave painting found in Lascaux, France dating to about 15,000 BC appears to depict the phases of the moon and could be considered the earliest known lunar ephemeris, but the earliest known planetary ephemeris appear to have been produced by the Babylonians around 523 BC

In early times, ephemeris appears to have been used to predict solar eclipses and, in some cultures, to attempt to predict the future. The positions of the planets in the sky, relative to the constellations and to each other, were often believed to have significance for Earth’s affairs: some configurations were considered favorable and others not. The configuration of the planets at the time of a person’s birth was also thought to influence that person’s character and destiny, and ephemeris was used to attempt to predict what was in store for people and society at large. They are still used this way by astrologers today; however, there are differences between an astrological ephemeris and an astronomical ephemeris. Astrological ephemeris usually give planetary positions on the ecliptic with respect to the vernal equinox and show the zodiacal constellation in which the planet resides on a given date.

The main elements of astronomical ephemeris data, for a given planet and date, are right ascension and declination. These can essentially be thought of as the horizontal and vertical positions of the object. Some may, however, provide more information. For example, the ephemeris published in the United States by the National Aeronautics and Space Administration (NASA) gives, among other things, the distance of the planet from the earth in astronomical units and its speed, in kilometers per second, relative to the Earth.

Modern ephemeris are used by professional and amateur astronomers to locate objects of interest, which can include planetoids and large asteroids as well as planets. The object can be searched in the object table for the requested date so that the user knows where to point the telescope. Ephemeris is also used to trace the paths of space probes used to explore the planets of the solar system. Global Positioning System (GPS) satellites broadcast ephemeris, providing their precise positions at regular intervals.

Since the motions of planets orbiting the Sun are very predictable, it is possible to compile ephemeris far into the future. The planets, however, are slightly influenced by the gravitational pull of many asteroids whose precise orbits are not known, so for very long periods the predicted positions may be slightly out of step with reality. For this reason the astronomical ephemeris tables are regularly revised.

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