Comments on the Effect of Adopting New Precession and Equinox Corrections

Abstract

The new astronomical constants being introduced for 1984 affect the derived values of UT1 and terrestrial longitudes differently for the space techniques than for the optical techniques. The new sidereal time equation has been defined to avoid a discontinuity in UT1 as obtained by the established techniques. However, this definition does introduce a discontinuity in both the terrestrial longitude system and the UT1 rate derived by the space techniques which have an inertial celestial reference frame rather than a stellar catalog frame. The use of consistent expressions for precession and Greenwich Mean Sidereal Time will not eliminate differences between the inertial and classical optical techniques. Only improvements in the accuracy of the precession constant and equinox offset and drift will bring about consistency. Improvements in the precession constant can be expected in the next few years. The inertial techniques will exhibit shifts in the derived UT1 rate and terrestrial longitude system with each change in the precession constant if Greenwich Mean Sidereal Time is referenced to the mean equinox of date, as is the present practice, but would be stable if the reference is a fixed equinox. The latter choice is recommended.