Rotation of the Planet Mercury

Abstract

SOLAR tidal friction must be an intense effect for Mercury, and it must be expected that the planet's spin would have relaxed from any original value to one that is under the control of this effect in a time short compared with the age of the solar system. The retarding torque exerted by the Sun on a planet is proportional to 1/r⁶ (where r is the distance Sun-planet), a factor which is some 300 times greater for Mercury than for the Earth. For a planet on a circular orbit the final condition would then be one of synchronous rotation like the motion of the Moon with respect to the Earth. Mercury's motion around the Sun takes 88 days and for synchronous rotation the sidereal period would thus be 88 days also. The observed value of 59 ± 5 days differs markedly from this (see preceding communication).