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The term "astronomical unit" was coined in 1903. Prior to 1938, the AU was the mean distance from the Earth-Moon systems center of mass to the Sun without the perturbation of a major planet, or the half-length diameter of an unperturbed elliptical orbit around the Sun.
In 1976, the International Astronomical Union issued a series of the most important units used in astronomical research, one of which is the distance between the Sun and the Earth known as the Astronomical Unit (AU). According to the original definition of the International Astronomical Union, the distance between the Sun and the Earth is "the radius of the orbit of a massless particle moving at an angular speed of 0.01720209895 radians per day in a circular orbit with the center of the Sun under the gravitational influence of the Sun." The published data at the time was 1 AU equal to 149597870.691 km.

In addition to the ambiguity of the definition itself, there is another problem that is difficult to accept: since it is a "fundamental unit", it seems that it should be a fixed number, but according to the 1976 International Astronomical Union definition, the astronomical unit is a constantly changing number. First, the mass of the Sun is decreasing, causing the astronomical unit to change slowly. Second, according to general relativity, the definition of spacetime is relative and is related to the spacetime in which the observer lives. According to the above definition, the astronomical units measured in different parts of the solar system will be different, for example, the astronomical units measured on Jupiter (the most massive planet in the solar system) will be more than 1,000 kilometers different from the astronomical units measured on Earth.