Motion of Phobos

Abstract

THERE have been several attempts to explain the large secular acceleration of the motion of Phobos (the closer of the two satellites of Mars) that was derived by Sharpless1 by fitting a quadratic expression for the mean longitude to five selected groups of orbital elements covering the period 1879 to 1941. An analysis2 of all suitable visual observations made during the period 1877–1929 failed, however, to confirm this secular acceleration. The orbital elements obtained in this analysis, taking the secular acceleration as zero, were subsequently3 shown to fit these visual observations and also positions obtained by Sharpless in 1941 and, photographically, by Kuiper in 1956 to within about ±2° in the value of the constant term in the linear expression for the mean longitude. This uncertainty is believed to be due partly to the errors of the observations and partly to the approximations in the orbital model used to fit the observations.