When I was in high school, I was taught that Mercury's orbit and rotation were locked 1:1. 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. Here's a link to a .gif animation of Mercury's rotation & orbit. https://commons.wikimedia.org/wiki/File:Orbital_resonance_of_Mercury.gif *I'm not 100% convinced that this particular gif animation is correct w.r.t. timing; it doesn't seem to follow Kepler's Second (Equal Area) Law, whereby equal areas are swept out in equal times, and thus Mercury should actually be moving faster when closer to the Sun.* https://en.wikipedia.org/wiki/Mercury_%28planet%29 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? In any case, when I was in high school, we talked about the "dark side of Mercury", still thinking that it had a 1:1 ratio -- like that of the Earth's Moon. In such a case, the "dark side" should be quite cold -- particularly since Mercury has essentially no atmosphere to transfer energy from the "bright side" to the "dark side". I would imagine that the temperature of the "dark side" in such a case would be determined by an equilibrium between the temperature of deep space and the *mean* temperature of Mercury (averaged over the entire body), because in that case, Mercury would be *conducting* heat from the bright side to the dark side solely by conducting the heat through the interior of the planet's body. However, there is actually no "dark side" of Mercury, which is a pity, because it might be an interesting place for various scientific measurements. I do hope that no science fiction stories were harmed in the making of the discovery that Mercury was not in 1:1 resonance!