Astronomical unit

[ti ā n wén d ā n wèi]

A unit in astronomy that measures the distance between celestial bodies

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The astronomical unit is astronomy A unit that measures the distance between celestial bodies. With A U. Is the average distance between the earth and the sun. International Astronomical Union In 1964, it was decided to adopt 1A U.=1.496x10 eight Km, from 1968 to the end of 1983; In 1978, it was decided to use 1A instead U. =149597870 km, used since 1984. This constant was measured by Solar parallax Calculated; Since the 1960s Radar astronomical observation , derived from the speed of light and the optical aberration per unit distance tA. Generally used for measurement solar system The distance between the celestial bodies in.^[1]

Chinese name: Astronomical unit
Foreign name: Astronomical Unit

Meaning: Length unit
Symbol: A.U.
Length: 149597870700 m

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The term "astronomical unit" appeared in 1903. Before 1938, the astronomical unit was defined as Megaplanets Perturbation (see Perturbation theory ）Down, from Earth moon system The average distance from the center of mass to the sun, or Earth moon system Perturbation free rotation of the center of mass around the sun Elliptical orbit Of Semimajor diameter 。^[2]

1976 International Astronomical Union A series of Astronomical research One of the most important units used is the Sun earth distance 。 According to the original definition of the International Astronomical Union, the distance between the sun and the earth is "the distance between Circular orbit At 0.01720209895 radians per day angular velocity Athletic Massless particle Orbital radius of ". The data released at that time was 1 astronomical unit equal to 149597870.691 kilometers.

Astronomical unit

The distance between the sun and the earth defined in this way is in addition to the definition itself Obscure In addition, there is a difficult problem to accept: since it is“ Basic unit ”It seems that it should be a fixed number, but according to the definition of the International Astronomical Union in 1976, the astronomical unit is a constantly changing value. First of all, the mass of the sun is decreasing, leading to a slow change in the value of the astronomical unit. Secondly, according to General relativity The definition of space-time is relative to Observer It is related to the time and space. As defined above solar system The values of astronomical units measured in different places within the solar system will be different. For example, the astronomical units measured on Jupiter (the most massive planet in the solar system) differ from those measured on the Earth by more than 1000 kilometers.

It is in order to solve this problem that the 28th IAU General Assembly issued the B2 resolution on August 30, 2012, which unanimously approved the change of the definition of astronomical units. It is stipulated that the length of the astronomical unit is 149597870700 meters, which is no longer a constantly changing value.