What is Mercury's specific spin-orbit resonance relative to its orbit around the Sun?
Answer
Three rotations for every two orbits around the Sun
Mercury exhibits a unique rotational characteristic known as a spin-orbit resonance, specifically locking into a $3:2$ resonance with the Sun. This means that for every two complete orbits Mercury makes around the Sun (which takes 88 Earth days per orbit), the planet completes exactly three full rotations on its axis. This slow rotation, relative to its orbital speed, is what contributes significantly to the extreme thermal differences experienced across its surface, resulting in days that are exceptionally long compared to its year, thereby exacerbating the temperature swing between its sunlit and dark sides.
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