Under Where is the Selk crater on Titan with respect to Saturn? there is @BrendanLuke15's comment which provides a helpful hint: Tidally locked moons have their 0° longitude defined as the 'sub-planet' point (In every instance I've seen) The first thing I think of is libration. (though when I w...

My understanding is that when tidal locking of a body first happens it transitions from a state where the average rotation period is faster than the orbital period to a state where they are the same on average. This leaves it "rocking" back and forth as seen from the other body by almost ±180° in...