concept

Tidal locking

The state in which a body spins exactly once on its axis for each orbit it makes, so it keeps the same face turned toward the object it circles. Gravity stretches the body into an egg shape, and friction in that tidal bulge slowly brakes its spin until rotation matches the orbit. The Moon is locked to Earth this way; in canon, Pandora is locked to Polyphemus - so its day is its month.