- Resonance effect by which bodies can match their spin periods to the orbital periods of other astronomical bodies. The most conspicuous example – visible even to the naked eye observer – is that of the Moon, which turns on its axis once per monthly orbit so the same face, that of the Man in the Moon, is always visible. Spin-orbit coupling arises from gravitation acting over long periods on small asymmetries like the non-spherical shape of the Moon. Some cases are more complex, such as the rotation of Mercury (three spins on its axis per two orbits of the Sun). Another remarkable case from the inner solar system is Venus’s day, which is equal in length to two thirds of an Earth year, so that the same face of Venus is pointed towards the Earth at each of its conjunctions. Similar couplings are observed in many cases in the outer solar system, a notable example being the orbital period of Charon, which matches the length of the day on Pluto. So Charon seems to hover above the same spot on Pluto’s surface, making it the solar system’s first geostationary satellite.