From Earth’s perspective, the full moon is the lunar period in which the Moon appears completely illuminated. This happens when the Earth is between the Sun and the Moon.
- The period between a full moon and the next repetition of the same phase, called a synodic month, is around 29.53 days on average.
- As a result, in lunar calendars where each month starts on the new moon’s day, the full moon occurs on the 14th or 15th day of the lunar month.
- Since a calendar month is made up of a fixed number of days, a lunar calendar month may be either 29 or 30 days long.