Ephemeris

In astronomy and celestial navigation, an ephemeris is a book with tables that show where stars, planets, and other objects in space are located at different times. These tables tell us the position and sometimes the speed of these objects in the sky. Long ago, these positions were printed in books at regular times, and figuring out these tables was one of the first jobs for early mechanical computers. Today, we often get this information electronically, but printed books are still useful when we don’t have computers.

The position of stars and planets from an ephemeris is usually given using a special way to describe places in the sky, called right ascension and declination. Ephemerides help us understand many interesting events in the sky, like eclipses, when planets seem to move backward, and the different phases of the Moon.

People who study the stars and navigate using the sky rely on ephemerides. Even tools like GPS signals use ephemeris data to find where satellites are in space. Astrologers also use these tables for their work.

History

An ephemeris is a special book that tells us where stars, planets, and other objects in space will be at certain times. People have used these books for thousands of years.

Long ago, in places like Babylon and Greece, people made lists showing where the Moon and planets would appear. Over time, many smart people from different countries made better lists. In the 1400s, a man named Regiomontanus made a book called "Ephemerides" in Germany. Even famous explorers like Christopher Columbus used these books to predict things like eclipses. Later, scientists like Johannes Kepler made even better lists using new ideas about how the planets move.

Modern ephemeris

For science, a modern planetary ephemeris is a special software program that can show the positions of planets, their moons, asteroids, or comets at almost any time the user chooses.

Since electronic computers were introduced in the 1950s, scientists have used a method called numerical integration to calculate these positions. One famous example is the Jet Propulsion Laboratory Development Ephemeris. These programs can calculate positions for many years into the past and future because scientists have good theories about how objects in space move. However, some details, like the effects of many asteroids, are still hard to predict exactly. NASA's Jet Propulsion Laboratory updates its ephemerides almost every year to include the latest information.

Ephemerides are very important for guiding spacecraft and for studying the planets, their moons, stars, and galaxies. They usually show where objects appear in the sky using special coordinates called right ascension and declination. They may also include extra details like how bright an object looks, how far away it is, and when it rises or sets in the sky.

Celestial navigation can be used as a backup when satellite signals are not available. Some navigation programs include their own ephemeris, while others use data from modern books like the Nautical Almanac.