Moon Phase Calculator

Everything is computed on your device from published formulas; nothing is downloaded. Your date, time and time zone are turned into one moment in Universal Time. Two methods from Jean Meeus's Astronomical Algorithms do the rest.

The share of the Moon that is lit comes from the angle between the Sun and Earth as seen from the Moon, called the phase angle i (chapter 48):

k runs from 0 at new moon to 1 at full moon. The Moon is waxing while the lit part grows and waning while it shrinks.

The times of new and full moon come from chapter 49. It starts from the average cycle of 29.5306 days, about 29 days and 13 hours, then adds corrections for the Moon's and Earth's oval orbits and small pulls from the planets. NASA's formula for the difference between atomic time and Earth's rotation turns the result into clock time. In tests the January 2024 full moon comes out at January 25 at 5:53 PM UTC, within a minute of the U.S. Naval Observatory's 5:54 pm.

The Moon at the Apollo 11 landing

The Eagle landed at 4:17 pm New York time on July 20, 1969. The Moon was a waxing crescent, 33% lit, 6.3 days after new moon. Mission planners wanted the Sun low over the landing site, so long shadows showed the shape of the ground.

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The blue moon of May 2026

In New York time, May 2026 has two full moons: May 1 at 1:23 PM and May 31 at 4:45 AM. The second full moon in one calendar month is called a blue moon. It happens about every two and a half to three years, because a month is a little longer than the 29.5-day cycle.

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A dark sky for the Perseids

The Perseid meteor shower peaks around August 12 each year. In Denver at 10:00 pm on August 12, 2026, the Moon is new moon, 0% lit, so no moonlight hides the fainter meteors. Compare a year when the peak falls near full moon, when only the brightest ones show.

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The phase name sums up the lit share and whether it is growing. The tool calls the Moon new below 2% lit and full above 98%, and uses the quarter names within 3 points of 50%. In between it is a crescent (under half lit) or gibbous (over half).

• Moon age is the days since the last new moon. Full moon comes at about day 15.
• Lit share is the same everywhere on Earth at one moment. Only the view turns: south of the equator a waxing Moon is lit on the left, so pick South to mirror the drawing.
• Dates depend on the time zone. The October 2026 full moon is October 25 at 9:11 PM in Los Angeles but October 26 at 1:11 PM in Tokyo. The four-zone card shows this for the next full moon.

• Treating a full "day" as the full moon. Full moon is an instant. The Moon looks full for a night or two either side, but the calendar date of the instant depends on your time zone.
• Forgetting the time of day. The lit share changes by up to about 11 points a day near the quarters, so a date with no time can be off. Set the time you will be looking.
• Expecting the same view everywhere. The phase is the same worldwide, but the Moon appears tilted near the equator and mirrored in the Southern Hemisphere.
• Mixing up waxing and waning. In the Northern Hemisphere, light on the right means waxing, growing toward full. Light on the left means waning.