A full moon occurs approximately every 29.5 days, which is the length of the lunar phase cycle, also known as a synodic month. This cycle represents the time it takes for the Moon to return to the same phase, such as from one full moon to the next.
Frequency of Full Moons
Full moons happen roughly once per calendar month due to the 29.5-day synodic cycle. Because this cycle is slightly shorter than most calendar months, each month typically contains one full moon. However, due to the mismatch between the lunar cycle and the Gregorian calendar, some years have 12 full moons while others have 13.
Extra Full Moons and Blue Moons
When a calendar year contains 13 full moons instead of 12, the extra full moon is often referred to as a "blue moon". This occurs approximately every two and a half years. A blue moon can also refer to the second full moon within a single calendar month, which is a rarer event.
Lunar Cycle Details
The 29.5-day period is not the same as the Moon’s orbital period around Earth, which is about 27.3 days. The longer synodic month accounts for the time needed for the Moon to realign with the Sun as seen from Earth, completing the full phase cycle. During a full moon, the Earth is positioned between the Sun and the Moon, causing the Moon’s near side to be fully illuminated [
