A full moon is often thought of as an event of a full night's duration. This is somewhat misleading because its phase seen from Earth continuously waxes or wanes (though much too slowly to notice in real time with the naked eye). Its maximum illumination occurs at the moment waxing has stopped. For any given location, about half of these maximum full moons may be visible, while the other half occurs during the day, when the full moon is below the horizon.
Many almanacs list full moons not only by date, but also by their exact time, usually in Coordinated Universal Time (UTC). Typical monthly calendars that include lunar phases may be offset by one day when used in a different time zone.
Full moon is generally a suboptimal time to conduct astronomical observations because the bright sunlight reflected by the Moon then outshines the apparently dimmer stars.
On 12 December 2008, the full moon occurred closer to the Earth than it had been at any time for the previous 15 years, called a supermoon.
On 19 March 2011, another full supermoon occurred, closer to the Earth than at any time for the previous 18 years.
On 14 November 2016, a full supermoon occurred closer to the Earth than at any time for the previous 68 years.
The date and approximate time of a specific full moon (assuming a circular orbit) can be calculated from the following equation:
where d is the number of days since 1 January 2000 00:00:00 in the Terrestrial Time scale used in astronomical ephemerides; for Universal Time (UT) add the following approximate correction to d:
where N is the number of full moons since the first full moon of 2000. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moon's orbit. See New moon for an explanation of the formula and its parameters.
The age and apparent size of the full moon vary in a cycle of just under 14 synodic months, which has been referred to as a full moon cycle.