An effect which allows observers on Earth to see almost 1° farther around the east and west limbs of the Moon's Earth-facing hemisphere. It results from the fact that the observer's position relative to the Moon changes between moonrise and moonset (see the diagram). The amount of diurnal libration depends on the observer's position on Earth, and can be up to 1°02′ for an observer at the equator.
diurnal libration: A slight change in an observer's position relative to the Moon during the course of a day
Subjects: Astronomy and Astrophysics.