WHAT IS THE ASTRONOMICAL UNIT OF LENGTH

This article proposes a definition of the astronomical unit of length, [AU], in the frame of general relativity. We argue that the so-called coordinate unit is not meaningful in relativity for it is generally n ot unique at a point of the space under measurement. All the coordinat es have numerical values only after the units of proper time and prope r length have been chosen. Consequently, we suggest that the astronomi cal units of time and length, [D] and [AU] respectively, be proper uni ts and do not depend on the choice of the coordinate system just as se cond and meter in the international system of units. On the other hand , the classical definition of [AU] by Kepler's third law is not suitab le any more because it uses a distance between two points and therefor e is coordinate dependent. We propose that [AU] should be defined so t hat the heliocentric gravitational constant, k(S), is the square of a fixed value, 0.01720209895[AU](3)[D](-2). The relation between the int ernational and astronomical unit system is such that 1[D] is equal to 86400 SI seconds and the relation between the length units is determin ed by a primary constant, tau(A), which is the light time of unit leng th. tau(A) is usually determined by a fitting of observational data. T he unit of the value of tau(A) should be the SI second. We also sugges t that all the astronomical constants in the IAU list should be proper quantities and therefore coordinate independent.