Full moon

A full moon is the lunar phase that occurs when the Moon is completely illuminated as seen from Earth. This occurs when Earth is located directly between the Sun and the Moon (more exactly, when the ecliptic longitudes of the Sun and Moon differ by 180 degrees). This means that the hemisphere of the Moon that is facing Earth (the near side) is almost fully illuminated by the Sun and appears round (while the far side is almost completely unilluminated). On some occasions at the time of full moon there is also a lunar eclipse so the moon's face appears reddish due to the rayleigh scattering of blue light in Earth's atmosphere.

Lunar eclipses can occur only at full moon, where the Moon's orbit allows it to pass through Earth's shadow. Lunar eclipses do not occur every month because the Moon usually passes above or below Earth's shadow (which is mostly restricted to the ecliptic plane). Lunar eclipses can occur only when the full moon occurs near the two nodes of the orbit, either the ascending or descending node. This causes eclipses to only occur about every 6 months, and often 2 weeks before or after a solar eclipse at new moon at the opposite node.

The time interval between similar lunar phases—the synodic month—averages about 29.53 days. Therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because calendar months have a whole number of days, lunar months may be either 29 or 30 days long.

A full moon is often thought of as an event of a full night's duration. This is somewhat misleading because the Moon seen from Earth is continuously becoming larger and/or smaller (though much too slowly to notice with the naked eye). Its absolute maximum size occurs at the moment expansion has stopped. For any given location, about half of these absolute maximum full moons will be potentially visible, as the other half occur during the day, when the full moon is below the horizon. Many almanacs list full moons not just by date, but by their exact time as well, usually in Coordinated Universal Time (UTC). Typical monthly calendars that include phases of the Moon may be offset by one day if intended for use in a different time zone.

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