Moon phase calculator
The Moon's phase on your birth date, how much of it was lit, and the phase on your birthday.
A moon phase calculator shows how the Moon looked on any date, including the day you arrived. Enter your birth date above to see the phase you were born under and what percent of the Moon was lit.
Below the main result you get a second line with the moon phase on your birthday. Your browser does all the math, so the date is not sent to anyone and is not stored.
The eight phases in brief
The Moon goes from dark to full and back in about 29.5 days. People split that cycle into eight named phases, and each one lasts a little over three and a half days. Waxing means the lit part is growing. Waning means it is shrinking.
| Phase | How much is lit | Traditional theme |
|---|---|---|
| New Moon | Almost none | Starting fresh |
| Waxing Crescent | Under half, growing | Gathering effort |
| First Quarter | Half, growing | Making a choice |
| Waxing Gibbous | Over half, growing | Fine tuning |
| Full Moon | Almost all | Seeing clearly |
| Waning Gibbous | Over half, shrinking | Sharing what you learned |
| Last Quarter | Half, shrinking | Letting go |
| Waning Crescent | Under half, shrinking | Rest and closing |
Astrologers read the phase at birth as a clue to your rhythm. Someone born near a New Moon has the Sun and Moon in about the same part of the sky. At a Full Moon the two stand opposite each other.
What the percent lit means
The percent shows how much of the Moon’s face caught sunlight as seen from Earth. A result of 50% falls at a quarter phase. A result of 98% with a growing Moon sits a day or so before full.
The number also helps when two phases look alike. A Moon that is 30% lit could be a waxing or a waning crescent. The calculator names which one it was.
Noon UTC and change days
The calculator works out the phase at noon UTC on your birth date. UTC is the world’s standard clock, so everyone who enters a given date gets the same answer.
Your own birth moment could fall up to half a day on either side of that noon, and sometimes more once your time zone is added. The Moon’s light changes by several percent in a day, and a phase can turn over during those hours. If you were born on a change day, your exact phase can differ from the one shown here. A baby born late in the evening in California, for instance, arrived on the next date by UTC.
The phase on your birthday
Your birthday comes back on the same date, and the Moon does not. Twelve lunar cycles add up to about 354 days, about 11 days short of a year. So the phase slips forward through the cycle each time.
About every 19 years the two calendars nearly line up again. Around your 19th and 38th birthdays, the Moon looks close to the way it did when you were born.
To see what the sky is doing now, open moon phase today. You can compare the current phase with your birth phase and count how far along the cycle it has moved. Some people like to note each time the Moon returns to their birth phase, which happens about once a month.
Questions
What moon phase was I born under?
Enter your birth date above and the calculator names the phase and the percent lit. It uses noon UTC on that date.
Why does another calendar show a different phase for my date?
A phase can change during a day. If you were born on a change day, a chart set to your time zone or birth hour can show the next phase.
Why is the phase on my birthday different from my birth phase?
The Moon's cycle lasts about 29.5 days and doesn't fit evenly into a year. The phase on your date slips by about 11 days each year.
Does the moon phase need a birth time?
No. A date gives a good answer on most days. Only a change day, when one phase turns into the next, depends on your birth hour.
Do you keep the date I enter?
No. The calculator runs in your browser, so we never see the date and no copy is kept.