Tidal Locking

Tidal locking happens when an orbiting body rotates exactly once per orbit, so the same side always faces the object it orbits.

Tidal locking occurs when a moon or planet spins exactly once for every orbit, so it always shows the same face to the body it circles.

The Moon is tidally locked to Earth, which is why we always see the same side of it. Tidal forces stretch an orbiting body slightly; friction from this stretching gradually slows its spin until it matches its orbit.

Planets in the habitable zones of red dwarf stars orbit so close that many are probably tidally locked to their stars, with permanent day on one side and permanent night on the other.