Full Moon Dates Calculator for Any Year

Full moon is an instantaneous geometric event: the moment when the apparent ecliptic longitude of the Moon exceeds that of the Sun by exactly 180 degrees. Successive full moons are separated by one synodic month, which averages 29.530588853 days, or 29 days 12 hours 44 minutes and 3 seconds. This calculator advances integer multiples of that mean value from a reference lunation near January 6, 2000, then converts the resulting Julian Day number back to a calendar date. Twelve synodic months total about 354.37 days, roughly 11 days short of a solar year, which is why a thirteenth full moon appears in a calendar year every two to three years and why the dates creep earlier from year to year. The mean-value approach is the main limitation. The true interval between consecutive full moons ranges from about 29.27 to 29.83 days, because the lunar orbit has an eccentricity near 0.055 and is strongly perturbed by the Sun. The dominant correction, the evection term identified by Ptolemy, reaches roughly 1.27 degrees in longitude, and the full-moon instant computed from mean lunations can therefore differ from the true one by up to about half a day. Since the event is timed in Universal Time, an instant late in the UTC day may fall on the previous or following local calendar date. Distance also varies between roughly 356,500 km at perigee and 406,700 km at apogee, which is the origin of the popular supermoon label rather than any change in phase.

Lunar prediction is among the oldest quantitative sciences. Babylonian scribes recorded lunar events systematically from the eighth century BCE and derived a mean synodic month accurate to a few seconds, encoded in the goal-year texts and in the 18-year Saros eclipse period. The 19-year cycle in which 235 synodic months nearly equal 19 solar years was known in Babylon and was described in Greece by Meton of Athens in 432 BCE. Hipparchus, working in the second century BCE, discovered the lunar inequality now called the equation of the centre, and Ptolemy added the evection in the Almagest around 150 CE. Newton's gravitational treatment in the Principia of 1687 turned these empirical corrections into a dynamical problem, and Ernest W. Brown's lunar theory, published between 1897 and 1908, supplied the tables used for national almanacs for much of the twentieth century. The naming layer is far more recent and much less standardised. The Wolf, Snow, Worm and similar month names were popularised by the Maine Farmers Almanac in the early twentieth century and are attributed to a mixture of Algonquian, colonial and European usages, with considerable variation between sources. The modern second-full-moon-in-a-month definition of a Blue Moon is a documented misreading: James Hugh Pruett described the Maine Farmers Almanac rule incorrectly in Sky and Telescope in March 1946, and the error spread widely after the magazine repeated it in 1980.