Secular Increase of the Astronomical Unit : a Possible Explanation in Terms of the Total Angular-Momentum Conservation Law

Aims. We show a possible explanation for the recently reported secular increase of the Astronomical Unit (AU) by Krasinsky and Brumberg (2004). Methods. The mechanism proposed is analogous to the tidal acceleration in the Earth-Moon system, which is based on the conservation of the total angular momentum and we apply this scenario to the Sun-planets system. Results. Assuming the existence of some tidal interactions that transfer the rotational angular momentum of the Sun and using reported value of the positive secular trend in the astronomical unit, d dt AU = 15±4 (m/cy), the suggested change in the period of rotation of the Sun is about 3 ms/cy in the case that the orbits of the eight planets have the same “expansion rate.” This value is suffi ciently small, and at present it seems there are no observational data which exclude this possibility. Effects of the change in the Sun’s moment of inertia is also investigated. It is pointed out that the ch ange in the moment of inertia due to the radiative mass loss by the Sun may be responsible for the secular increase of AU, if the orbital “expansion” is happening only in the inner planets system.