Cadet Competition Written Practice Exam 2026 - Free Practice Questions and Study Resources.

A Moon day, which is defined by the period it takes for the Moon to complete one full rotation on its axis relative to the Sun, lasts approximately 29.5 Earth days. This duration is known as a synodic month, which accounts for the time necessary for the Moon to transition through all its phases, from new moon to new moon.

Understanding this synodic month is crucial in lunar studies and understanding the lunar cycle, as the Moon's rotation and orbital period do not synchronize with the Earth's day. While the Moon's actual rotation period (the sidereal day) is about 27.3 days, the synodic month is longer due to the Earth’s movement around the Sun. Thus, the Moon takes about 29.5 days to return to the same position relative to the Earth and Sun, aligning with the complete lunar phase cycle observed from Earth.