Planetary Ephemerides

Abstract

In the past twenty years there has been a great amount of growth in radiometric observing methods, as well as in classical optical observations. Through radar ranging and Doppler observations of the planets and spacecraft, we have been able to improve our knowledge of the location and motion of the planets by several orders of magnitude and have succeeded in planning and executing space missions which would have been difficult if not impossible to plan and to perform utilizing the classical ephemerides. We will outline the goals and methods employed by the Jet Propulsion Laboratory in its effort to develop improved ephemerides which accurately reflect the motions of planets in an inertial system.

We will demonstrate that in the ideal situation a radar observation is largely independent of the problems usually associated with the precessional motion of the earth’s axis and is, in fact, a reliable method for obtaining inertial mean motions. Based upon our hypothesis that the modern JPL ephemerides are valid in a fixed (i.e., non-rotating) coordinate system, we will explore the implications concerning optical observations of the Sun, precession, equinox drift and the relationship between dynamical and universal time scales, as well as comparisons with Newcomb (1898) ephemerides.