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Numerical tables containing the coordinates of the stars (planets, satellites, small planets, comets, etc.) and other elements referring to regular and successive periods of time, thanks to which it is possible to calculate the positions of the stars themselves in the heaven.
The name of ephemeris derives from the ancient Greek "efemeris" (everyday).
Both solar and sidereo time have small inaccuracies due to the irregularities of the Earth's rotation around its axis, which shows variations of 1 or 2 seconds per year. In addition, the Earth is gradually slowing down, and the period of its orbit increases by about one thousandth of a second every 100 years. Some of these variations can be taken into account; in other cases it cannot be done due to their irregular nature.
In 1940, these difficulties were eliminated with the introduction of ephemeris time, used mainly by astronomers when they need the highest precision to calculate the positions of planets and stars.
The time of ephemeris is based on the annual rotation of the Earth around the Sun, and its base position, as in the sidereal time, is the spring equinox. The use of mathematical tables allows converting ephemeris time to average solar time.
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