What is the astronomical unit of length?
Abstract
This article proposes a definition of the astronomical unit of length, [AU], in the frame of general relativity. We argue that the socalled coordinate unit is not meaningful in relativity for it is generally not unique at a point of the space under measurement. All the coordinates have numerical values only after the units of proper time and proper length have been chosen. Consequently, we suggest that the astronomical units of time and length, [D] and [AU] respectively, be proper units and do not depend on the choice of the coordinate system just as second and meter in the international system of units. On the other hand, the classical definition of [AU] by Kepler's third law is not suitable any more because it uses a distance between two points and therefore is coordinate dependent. We propose that [AU] should be defined so that the heliocentric gravitational constant, k_S_, is the square of a fixed value, 0.01720209895[AU]^3^[D]^2^. The relation between the international and astronomical unit system is such that 1[D] is equal to 86400 SI seconds and the relation between the length units is determined by a primary constant, τ_A_, which is the light time of unit length. τ_A_ is usually determined by a fitting of observational data. The unit of the value of τ_A_ should be the SI second. We also suggest that all the astronomical constants in the IAU list should be proper quantities and therefore coordinate independent.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 June 1995
 Bibcode:
 1995A&A...298..629H
 Keywords:

 REFERENCE SYSTEMS;
 RELATIVITY