The word "resonance" is used when periods are in in low integer ratios like 2:3. I have long heard that many of the planets' years are in low integer ratios, and that this is because this makes for a relative minimum of potential energy. —Dan Henry wrote: ----- I recently discovered (to my chagrin) that NASA's knowledge had improved, but mine had not; Mercury actually rotates 1.5x faster that it's orbital period -- i.e., two full elliptical orbits = 3 full rotations. ... Astronomers call Mercury's orbit/rotation a "resonance", but I don't understand why. The whole point of a 1:1 orbit/rotation locking is to eliminate the tidal effects on the planet, but any other ratio still produces tides. I'm not sure what effect is causing a "lock" to the 3:2 ratio; perhaps it is related to the excessive eccentricity of Mercury's orbit? ... -----