Moon Phase (Lunar phase) refers to the appearance of the illuminated
portion of the Moon as seen by us. The Moon phases vary cyclically
as the Moon orbits the Earth, according to the changing geometry
of the Earth, Moon, and Sun. The time between successive occurrences
of the same phase (e.g. full moon) is about 29.53 days (29 days,
12 hours, 44 minutes) on average.
Because this time is not an constant, making the moon phase shown
on an mechanical watch to be accurate is impossible. Therefore,
adjustment is required.