The Moon turns always the same hemisphere toward the Earth.
Observations of Venus and Mercury are prevailingly to the
effect that those planets always turn the same hemispheres
toward the Sun. Many, and perhaps all, of the satellites of
Jupiter and Saturn seem to turn the same hemispheres always
toward their respective planets. This widely prevailing
phenomenon is no doubt due to a widely prevailing cause, which
astronomers have all but unanimously attributed to tidal
action.