Cosmic spinning tops

Abstract

Poets, painters and musicians have often been inspired by the Moon, and their writings, paintings and songs dedicated to our satellite raise emotions and let the imagination run free. Yet every time we take a glimpse at the full Moon, it always shows us the same face, and this is even more evident when observing the lunar surface through a telescope. It is well known to astronomers that the Moon always shows the same hemisphere to the Earth and that this is due to the long-term gravitational interaction between the Moon and our planet. Tidal evolution is responsible for the peculiar relationship between the rotation of our satellite about its own axis and its orbital motion around the Earth, which causes the Moon never to turn its back on us. According to celestial mechanics, this is a classical example of a spin-orbit resonance.