  The radiative and particulate loss of mass by the Sun, -9.13*10^-14 Solar
masses per year or more causes the orbits of the planets to expand at the same
rate, and their periods to lengthen at twice this rate. Unfortunately, under
the present definition of the Astronomical Unit (AU) based on the fixed
Gaussian gravity constant kGS = 0.01720209895 (AU)^1.5/day, the value AUmet of
the AU in meters must decrease at 1/3 this rate, all these rates being
expressed logarithmically. The progress of the planets along their orbits slows
quadratically with time. For example, in one century Mercury would lag behind
the position predicted using constant solar mass by almost 1.4 km, in two
centuries 5.5 km. The value of AUmet can be made constant by redefining it,
based on a reference solar mass unit, such as the solar mass at J2000; else,
the solar Gaussian gravity constant kGS used in defining the AU could be
redefined proportional to the square root of the solar mass. Improved accuracy
of the ephemerides would impose useful bounds on losses due to axion emission
(Sikivie 2005). With no axion emission the Earth's semi-major axis grows 1.37
m/cy; with the maximum allowable such emission the result is 1.57 m/cy. Under
reasonable assumptions about alternate gravity theories, radar delay data are
used to show that the effect of a changing Newtonian gravity constant is
negligible.
