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Calculated from the mean lunar cycle of 29.53 days, accurate to within about a day: real lunar months vary slightly because the Moon's orbit is elliptical. Illumination is the sunlit fraction visible from Earth.
What The Moon Phase Calculator Works Out
- Moon phase: the Moon repeats its phases every 29.53 days (the synodic month), so any date's phase falls out of counting the days since a known new moon and taking the remainder. The "age" locates the phase: waxing through crescent, first quarter (day 7.4) and gibbous to full at day 14.8, then waning back to new. Illumination follows a cosine of that age, which is why the Moon lingers near full for two or three nights but sprints through the quarters.
- Moon age: days since the last new moon. The mean-cycle method is accurate to about a day because the Moon's elliptical orbit lets individual lunations run more than half a day long or short of the average.
- Next full moon: the approximate date the age next reaches 14.8 days. Full moons rise opposite the setting Sun, so the low golden moonrise is the photograph.
- Next new moon (advanced): the darkest skies of the month and the best window for stargazing and meteor showers; the Perseids in August and Geminids in December are much better in a moonless week.
- Next quarter moon (advanced): whichever of first or last quarter comes first. The half-lit Moon's slanting light shows craters and mountains that the flat-lit full Moon hides, so it is the best phase for a small telescope.
- Name of the next full moon (advanced): the traditional names popularised by almanacs: Wolf Moon in January through Harvest Moon in September and Cold Moon in December. A "blue moon" is simply a second full moon in one calendar month, possible because 29.53 days undercuts every month but February; February can conversely miss a full moon entirely.
- Next spring tides (advanced): the biggest tidal ranges come a day or two after full and new moon, when Sun and Moon pull in line; the smallest (neaps) follow the quarters. Anglers, coastal walkers and anyone planning a beach day price the fact into their plans.
- Which side is lit (advanced): from the UK the waxing Moon shows its lit side to the right and sets in the evening sky; the waning Moon is lit on the left and rules the pre-dawn hours. The southern hemisphere sees it the other way round, and the seasonal moon names shift by six months.
- Table of upcoming phases (advanced): the next 12 (or up to 36) full moons with their names and any blue moons flagged, the next new moons with a dark-sky window either side, or every main phase in date order.