Moon Phase Today

This calculator finds the moon phase the way an almanac does. It computes the instants of the four principal phases (new moon, first quarter, full moon and last quarter) with the method in chapter 49 of Jean Meeus's Astronomical Algorithms (2nd edition, 1998). That is a mean lunation, then 25 periodic terms for the positions of the Moon and the Sun, then 14 small terms for the pull of the planets. The result is in Terrestrial Time, the uniform time scale astronomers compute in. UTC has run 69.184 seconds behind it since the leap second at the end of 2016, so the calculator subtracts 69 seconds and rounds to the minute.

The four principal phases are instants. The other four names cover the days between them: waxing crescent from new moon to first quarter, waxing gibbous from first quarter to full, then waning gibbous and waning crescent back to new. A date gets a principal phase name when that instant falls within it, read in UTC. Every other date takes the name of the stretch it is in. The lit percentage and the moon's age are taken at 12:00 UTC on the date, which is 7 or 8 a.m. on the US East Coast.

The lit percentage uses the low-precision formula in Meeus's chapter 48, which needs only the Moon's mean elongation from the Sun and the mean anomalies of the Sun and Moon. On his worked example for April 12, 1992 it gives 68.0% against 67.9% from the full method. It ignores how far the Moon sits north or south of the Sun's path, so it reads 100% at every full moon, where the true figure can be as low as about 99.8%.

The cycle of phases is the synodic month, 29.530588 days on average (Meeus uses 29.530588861). It is longer than the sidereal month, the 27.32 days the Moon takes to circle Earth once against the stars. In that time Earth moves about 27 degrees along its orbit, and the Moon needs a little over two more days to line up with the Sun again.

No two lunations are the same length. The Moon's orbit is an ellipse and the Moon moves fastest near perigee, and Earth's speed around the Sun changes through the year as well. In Meeus's formula the largest correction to a new moon, from the Moon's position in its orbit, is worth up to 0.41 days, almost 10 hours, and the Sun's term adds up to about 4 hours more. Between 1900 and 2100 this method gives lunations from 29.27 to 29.83 days long and gaps between successive principal phases from 6.58 to 8.24 days, so the phases do not keep to a weekly schedule.

Full moon is one instant, yet the Moon looks full for about three nights. The lit fraction changes slowly near full: a day and a half either side, at least 96% of the disk is still lit. Near the quarters it changes fastest, by up to about 12 percentage points a day.

The phase depends only on where the Sun, Earth and Moon are, so it happens at the same instant for everyone and the lit fraction is practically the same everywhere on Earth. The date of a phase depends on your time zone. The full moon of October 26, 2026 at 04:12 UTC is 12:12 a.m. on October 26 in New York and 9:12 p.m. on October 25 in Los Angeles. The orientation changes too: from the Northern Hemisphere a waxing moon is lit on the right, and from the Southern Hemisphere on the left.

The current moon phase here is a reading for one instant, 12:00 UTC, and the moon phase tonight is what you see after dark. In the US, evening falls between this calculator's readings for today and tomorrow, so the lit percentage you see sits between the two. When a principal phase lands overnight, the Moon that evening is within hours of it, and it looks the same either side of the exact time.

This calculator does not give moonrise or moonset, which depend on your latitude and longitude, and it does not predict eclipses.