In case it isn't clear from Dan's explanation, the potential energy is locally minimized when Mercury's "gravitational axis" is pointing through the center of the Sun at perihelion. Other inner-system weirdness: at least when I was a young adult, it was believed that Venus was in a tidal lock with the Earth, rotating slowly in retrograde so that at each pass of the Earth, Venus would keep one side facing us as it went past. On Tue, Jan 3, 2017 at 12:17 AM, Dan Asimov <asimov@msri.org> wrote:
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?
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