> But there is one planet where the calendar would need zero finessing: Mercury. The small planet revolves exactly three times, or days, over the course of two years — allowing its calendar to naturally align every other year.
I would imagine that this is some form of resonance, like the tidal locking of the moon, but i can't explain it myself.
It is indeed:
> In some cases where the orbit is eccentric and the tidal effect is relatively weak, the smaller body may end up in a so-called spin–orbit resonance, rather than being tidally locked. Here, the ratio of the rotation period of a body to its own orbital period is some simple fraction different from 1:1. A well known case is the rotation of Mercury, which is locked to its own orbit around the Sun in a 3:2 resonance.
Or is that just tidal locking? I know the moon does something similar but there it seems like revolution time = rotation time, not perfect multiple.