If I may....
I thought that Mercurrrry was tidally locked with the Sun.
Something like that, but not a direct hemispheric "lock".
Mercury rotates every 58.6 days and orbits every 88 days.
88/58.6 = 3 / 2 (give or take)
It rotates an integer number of times (3) for an integer number of orbits (2).
It's a dynamic thing, these resonances show up all over the solar system in various forms.
Reading assignment for tomorrow, the gaps in the rings of Saturn.