astronomical unit

A unit of distance, roughly equal to the average distance from the earth to the sun, about 149,600,000,000 meters. It is defined as the radius of a circular orbit in which a body of negligible mass, and free from perturbations, would revolve around the sun in 2π/k days, where k is the Gaussian gravitational constant. Symbol, ua; formerly A, AU or a.u..

In 1976 the 16th General Assembly of the International Astronomical Union defined the astronomical unit of length (as part of the International System of Astronomical Constants) as “that length for which the Gaussian gravitational constant (k) takes the value 0.0172 020 989 5 when the units of measurement are the astronomical units of length, mass and time.”¹

The astronomical unit of time is the day, defined as 86,400 seconds (the second as defined in SI), and the astronomical unit of mass is the mass of the sun, by definition 1.9891 × 10³⁰ kilograms. The value chosen for the Gaussian gravitational constant makes an AU approximately the earth-sun distance; expressing the value of the AU in meters makes one AU = 1.495 978 70 × 10¹¹ meters.

In the preparation of ephemerides in the 1990s, it was found necessary to use the value 1.495 978 706 6 × 10¹¹ meters.

In its early days the astronomical unit was not nearly so precisely defined. In 1922, the Commission des notations, des unités et de l'économie des publications of the International Astronomical Union accepted, and the First General Assembly adopted, the recommendation of an American committee² that 1) dimensions of celestial bodies, velocities, etc. be described in kilometers, 2) "la distance moyenne du soleil à la terre, ou unité astronomique" [the average distance between the sun and earth, or astronomical unit] be used for distances in the solar system and 3) the parsec be used for distances to stars.³

1.
International Astronomical Union.
Transactions of the International Astronomical Union, XVIB 58 (1977).

2.
Proceedings of the National Academy of Sciences, volume 6, page 360 (1920)

3.
A. Fowler, ed.
First General Assembly, held at Rome, May 2nd to May 10th, 1922.
Transactions of the International Astronomical Union. Volume 1.
London: Imperial College Bookstall, no date [1922 ?].

Pages 23, 138, 207.