IAU (1976) System of Astronomical Constants
The International Astronomical Union at its XVIth General Assembly in Grenoble in 1976, accepted System of Astronomical Constants. In 1994
the IAU recognized that the parameters became outdated, but retained the 1976 set for sake of continuity, but also recommended to start maintaining a set of "current best estimates"
;
this "sub group for numerical standards" had published a list, which included new constants
The system of constants was prepared
by Commission 4 on ephemerides led by P. Kenneth Seidelmann.
At the time, a new standard epoch was accepted; followed later
by a new reference system with fundamental catalogue, and expressions for precession of the equinoxes,
and in 1979 by new expressions for the relation between Universal Time and sidereal time
, and in 1979 and 1980 by a theory of nutation
. There were no reliable rotation elements for most planets, but a joint working group on Cartographic Coordinates and Rotational Elements was installed to compile recommended values
Units
The IAU system is based on the astronomical system of units:- The astronomical unit of time is the day of 86,400 SI seconds, which is close to the mean solar day of civil clock time.
- The astronomical unit of mass is the mass of the Sun.
- The astronomical unit of length is known as the astronomical unit, which in the IAU system is defined as the length for which the gravitational constant, more specifically the Gaussian gravitational constant k expressed in the astronomical units, takes the value of . This astronomical unit is approximately the mean distance between the Earth and the Sun. The value of k is the angular velocity in radians per day of an infinitesimally small mass that moves around the Sun in a circular orbit at a distance of 1 AU.
Table of constants
Other quantities for use in the preparation of ephemerides
| 3. | Equatorial radii |
| Object | Equatorial radius |
| Mercury | 2 439 ±1 |
| Venus | 6 052 ±6 |
| Earth | 6 378.140 ±0.005 |
| Mars | 3 397.2 ±1 |
| Jupiter | 71 398 |
| Saturn | 60 000 |
| Uranus | 25 400 |
| Neptune | 24 300 |
| Pluto | 2 500 |
| Moon | 1 738 |
| Moon's disk, ratio to Earth's equatorial radius | k = 0.272 5076 ae |
| Sun | 696 000 |