Probing gravity to the second post-Newtonian order and to one part in 10 to the 7th using the spin axis of the sun

Abstract

The close alignment of the Sun's spin axis with the solar system's planetary angular-momentum vector after 5 billion yr implies that the post-Newtonian gravitational interaction is Lorentz invariant to high precision. A metric gravity first post-Newtonian order (PPN) coefficient α2 is constrained to be of order 10-7 or less, and second post-Newtonian order (1/c4) aspects of the gravitational interaction are modestly constrained by the present-day solar system configuration. Observations related to possible precession of the spin axes of fast pulsars also are relevant to this precision test of the Lorentz invariance of gravity.