Moon Phase Calculator
See the Moon as it was on the night you were born, on a wedding day, or tonight. Phase, percentage lit, Moon age and the next full and new moon, worked out in your browser.
For entertainment and self-reflection: The astronomy here is real and checkable; any meaning people attach to a birth Moon is folklore and personal symbolism. Nothing on this page is a prediction, and it is not financial, medical, psychological or legal advice. Use it as a prompt for thought, not as a basis for decisions.
Find the Moon phase for any date
How the Moon phase is calculated
The Moon does not make its own light. What we call its phase is simply how much of the sunlit half we can see from Earth, and that depends on one angle: how far the Moon sits from the Sun in the sky, measured along the ecliptic. Astronomers call that angle the elongation. When the Moon is in the same direction as the Sun (elongation near zero) we see the dark side and call it a new moon. When it is opposite the Sun (about 180 degrees) the whole face is lit and we have a full moon. The quarters fall at 90 and 270 degrees, when exactly half the disc is bright.
This calculator does that geometry in two steps, both taken from Jean Meeus's Astronomical Algorithms, the standard reference used by planetarium software and almanac makers. First it finds the exact moments of the principal phases (new, first quarter, full, last quarter) around your date using Meeus's chapter 49 method, which starts from the average 29.53-day cycle and then adds several dozen periodic corrections for the Moon's elliptical orbit, the Sun's pull and the Earth's changing distance from the Sun. Second, it works out the illuminated fraction for your chosen moment from the Moon's mean elongation and anomalies (Meeus chapter 48). The result is converted from the uniform time scale astronomers use for orbital calculations into ordinary clock time with NASA's published formula for the difference between the two, called Delta T.
How accurate is that? We tested the phase times against NASA's century-long table of lunar phases by Fred Espenak: across roughly 2,500 new, quarter and full moons between 1950 and 2050, every computed moment landed within one minute of NASA's published time, and the typical difference was well under a minute, which is the rounding in NASA's table itself. The percentage lit agrees with a modern full ephemeris to within about a third of a percentage point.
Reading your result
The phase name follows the convention used by almanacs. New moon, first quarter, full moon and last quarter are instants, not days, so the calculator only shows one of those four names if the exact moment falls on the calendar day you picked, in your device's time zone. Every other day gets one of the in-between names: waxing crescent, waxing gibbous, waning gibbous or waning crescent. That is why a date that looked "full" to the eye can come back as waxing gibbous at 99 percent: the true full moon happened a few hours after midnight.
The Moon age is the number of days since the last new moon. It runs from 0 to about 29.5, and because the Moon's orbit is not a circle, a single lunar month can be several hours shorter or longer than the average. The illuminated percentage is the share of the visible disc that is sunlit. A first or last quarter is 50 percent; a crescent is anything below that; a gibbous Moon is anything above it.
The phase itself is the same for everyone on Earth at the same instant. What changes with location is the calendar date (someone in Tokyo may already be on the next day) and the orientation: from the Southern Hemisphere the lit side of a waxing Moon appears on the left rather than the right. The drawing above shows the Northern Hemisphere view.
Worked example: the night of the Moon landing
Enter July 20, 1969 with a time of 4:17 PM in Eastern Daylight Time (20:17 UTC, the moment the lunar module touched down). The calculator returns a waxing crescent, about a third lit, roughly six and a quarter days after the new moon of July 14. The next full moon came on July 29. That matches the photographs: the Apollo 11 crew landed on the Sea of Tranquility in the lunar morning, with long shadows, because the Sun had only recently risen over that part of the Moon. Mission planners chose the date partly for exactly that low lighting, which made the terrain easier to read on approach.
Try your own birthday next. Then try the same birthday in later years. Because twelve lunar months are about eleven days shorter than a calendar year, the phase on your birthday slides backwards through the cycle each year. It comes back to almost exactly the same phase every 19 years, a rhythm known as the Metonic cycle after the Athenian astronomer Meton, who described it in 432 BC. Your 19th, 38th and 57th birthdays tend to share the Moon you were born under.
A short history of watching the Moon
The Moon was humanity's first calendar. Notched bones from the Upper Palaeolithic have been read by some archaeologists as tallies of lunar phases, and the oldest named months in Mesopotamia, Egypt, China and the Americas all began with the first sighting of a thin crescent after new moon. The Babylonians kept systematic records of lunar observations for centuries and could predict the length of each month with impressive accuracy. The Islamic calendar remains purely lunar today, and the Hebrew and Chinese calendars are lunisolar, adding an extra month every few years to stay in step with the seasons. Easter, Passover, Ramadan, Diwali, the Mid-Autumn Festival and Lunar New Year all still move with the Moon.
The familiar full moon names, such as the Wolf Moon in January or the Harvest Moon nearest the September equinox, come mostly from colonial-era American and European almanacs, which drew on a mix of Algonquian, Anglo-Saxon and farming traditions. They are cultural labels rather than astronomy, and different Indigenous nations used many different names. The calculator shows the common almanac name for the month in which the next full moon falls.
Using the Moon for planning and reflection
There are good practical reasons to know the phase. Photographers and stargazers plan around it: the week around new moon gives the darkest skies for the Milky Way and meteor showers, while a full moon floods the landscape with light. Anglers, hunters and gardeners in many traditions time their work by the Moon, and tides are strongest (spring tides) near new and full moon. Hikers in the desert around Las Vegas often schedule night walks near full moon to avoid summer heat.
For reflection, many people simply enjoy knowing the sky they were born under, or marking anniversaries with it. A popular gift idea is a print of the Moon on a wedding day or a child's birth date. Some people use the cycle as a monthly rhythm for journaling: setting an intention at the new moon and looking back at the full moon. There is no evidence that the Moon's phase shapes personality or that births spike at full moon (large hospital studies have looked and found no meaningful effect), but a regular, visible cycle can be a useful nudge to pause and take stock.
Limits of this calculator
The calculator covers 1900 to 2100, where the method and the clock-time correction are well tested. It uses the time zone of your device when deciding which calendar day a phase falls on, so if you were born abroad, set your computer's clock or allow for the difference. It does not show eclipses, moonrise or moonset, which depend on your exact location, or the Moon's zodiac sign, which is a separate astrological calculation.
Frequently asked questions
What phase was the Moon when I was born?
Enter your date of birth, and your birth time if you know it, then press Show the Moon. You will see the phase name, the percentage lit and the Moon's age. The time only matters if you were born within about a day of an exact new, quarter or full moon.
Why does a full moon look full for three nights?
Full moon is a single instant, but the illuminated share changes slowly near it: about 99 percent a day either side and still around 95 percent two days out. The eye cannot tell 96 from 100 percent, so the Moon looks full for two or three nights.
Is the Moon phase the same everywhere in the world?
Yes. The percentage lit is the same for everyone at the same instant. The local date can differ, and in the Southern Hemisphere the Moon appears flipped, with the lit side of a waxing Moon on the left.
How accurate is this Moon phase calculator?
Phase times follow Meeus's Astronomical Algorithms. In our tests against NASA's table of lunar phases from 1950 to 2050 they matched within one minute; the percentage lit matched a full ephemeris within about 0.3 percentage points.
How often does my birthday fall on the same Moon phase?
About every 19 years. Nineteen calendar years are almost exactly 235 lunar months (the Metonic cycle), so the Moon on your 19th and 38th birthdays usually looks very like the Moon on the day you were born.
Do more babies arrive at full moon?
Large studies of hospital birth records have not found a meaningful full-moon effect. The idea persists because busy nights are memorable when the Moon is bright, a classic case of noticing hits and forgetting misses.
What is a blue moon?
In modern usage, the second full moon in a single calendar month. An older almanac definition is the third full moon in a season that has four. Either way it happens roughly every two to three years.
Sources and further reading
- Meeus, J. Astronomical Algorithms, 2nd ed., Willmann-Bell, 1998, chapters 47–49.
- Espenak, F. "Phases of the Moon: 2001 to 2100", NASA Goddard Space Flight Center, eclipse.gsfc.nasa.gov (used to test this calculator).
- Espenak, F. and Meeus, J. "Polynomial expressions for Delta T", NASA Eclipse Web Site.
- Wikipedia: "Lunar phase", "Metonic cycle", "Full moon" (naming traditions).
- NASA Science: "Moon Phases" overview, science.nasa.gov.
By Stanley King · About 8calc · Last reviewed October 2026