What is an Astronomical Unit (AU)?

Astronomical unit is a unit in astronomy that measures the distance between celestial bodies. Expressed in AU, its value takes the average distance between the earth and the sun. In 1964, the International Astronomical Union decided to use 1A.U. = 1.496x10 ⁸ kilometers, from 1968 to the end of 1983; in 1978, it decided to switch to 1A.U. = 149,597,870 kilometers, which has been used since 1984. This constant was calculated from the measured solar parallax before the 1960s; after the 1960s, it was derived from the speed of light and the unit distance light travel difference tA based on radar astronomical observations. Generally used to measure the distance between celestial bodies in the solar system. ^[1]

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The term "astronomical unit" appeared in 1903. Before 1938, astronomical units were used when there was no major planetary perturbation (see

1 astronomical unit = 1.495978707 × 10 ¹¹ meters = 149,597,870.7 kilometers = 92,960,000 miles = 490,800,000,000 feet