Universal time: Results from lunar laser ranging

Lunar laser ranging observations from the McDonald Observatory have been analyzed by least squares to estimate universal time (UT) simultaneously with parameters representing the locations of McDonald and the lunar retroreflectors, the orbits of the earth and the moon, and the moon's physical libration. The root-mean-square (rms) of the postfit range residuals for the 5-year period from October 1970 to November 1975 is 28 cm. The rms of the differences between our determinations of UT and those of the Bureau International de l'Heure is 2.1 ms, after removal of the mean difference. The differences between our determinations and those of Stolz et al. (1976), who analyzed most of the same ranging data with a different approach, have an rms of 0.8 ms and, in addition to high-frequency ‘noise,’ exhibit low-frequency variations of ≲ 1 ms in magnitude which result at least in part from differences in models used for the lunar orbit. Thus problems in modeling the moon's motions, especially those of long period, make difficult the determination of UT with the accuracy inherent in the ranging observations.