Your Moon Phase
at Birth

“What phase was the Moon in when I was born?” has two common answers, and they do not always agree.

The first is the sky's. Almanacs and calendars mark four instants in each lunar cycle, new moon, first quarter, full moon and last quarter, and call the days between them waxing or waning crescents and gibbous moons. That answer describes what you would have seen: a thin bow after sunset, a half Moon at dusk, a full Moon rising at nightfall.

The second is astrological. Dane Rudhyar's The Lunation Cycle (1967) cut the cycle into eight equal slices of 45° by the Moon's lead over the Sun along the zodiac, starting at the conjunction, and gave each slice a lunation type: New Moon, Crescent, First Quarter, Gibbous, Full Moon, Disseminating, Last Quarter and Balsamic. Each type lasts a little under four days, so it usually spans two sky names.

The two names clash most at the start of each slice. A Moon 30° past the Sun, two or three days old, is a crescent anyone can see, but Rudhyar's scheme still calls it a New Moon type, because the slice runs to 45°. The calculator shows both, labeled, so you can use whichever answer you came for.

The calculator sends the birth date, clock time, time zone and place to the API, which converts the time to Universal Time with the IANA time zone database and computes the positions of the Sun and Moon with the API's ephemeris. Four quantities describe the phase, and they are easy to mix up:

• The Moon's lead over the Sun in longitude, from 0° to 360°: the Moon's ecliptic longitude minus the Sun's. New moon is 0°, first quarter 90°, full moon 180°, last quarter 270°. Rudhyar's types are slices of this number.
• The elongation, the true angle between the Moon and the Sun in the sky, from 0° to 180°. It includes the Moon's latitude, up to about 5° north or south of the ecliptic, so it is never smaller than the longitude difference folded into that range.
• The phase angle, the angle at the Moon between the directions to the Sun and to the Earth: close to 180° at new moon and close to 0° at full.
• The lit fraction of the disk, which follows from the phase angle. The API takes the elongation, phase angle and fraction from the ephemeris's planetary-phenomena calculation rather than estimating them from the longitude difference.

For the principal phases either side of the birth, the API searches for the moments when the longitude difference reaches 0°, 90°, 180° or 270°, halving the search window until it is shorter than a second. We compared the result with the U.S. Naval Observatory's published table of Moon phases, which gives times to the minute, for every phase in 7 years from 1900 to 2050: all 346 agree within 50 seconds, and 302 within 30.

The naming rule

The sky name follows the almanac convention: if the birth falls on the same local calendar day as a new moon, first quarter, full moon or last quarter, the day takes that name; any other day is a waxing or waning crescent or gibbous, by which quarter of the cycle the Moon is in. “Local” means the birthplace's own date, so one instant can fall on two different days. The full moon of September 7, 2025, at 18:09 UT, was 11:09 AM on September 7, 2025, in Los Angeles and 7:09 PM in London, but 3:09 AM on September 8, 2025, in Tokyo. A birth that evening in Tokyo was not a full-moon birth by this rule; one the next day was.

The lunation type uses the longitude difference at the birth moment. If you leave the birth time blank, the calculator uses noon; the sky name is unaffected, and the result warns you when the type could have been the neighboring one at another time of day. The Moon gains 10.7° to 14.4° a day on the Sun, so that warning covers anything within 7.2° of a boundary.

The drawing on the result uses the lit fraction and whether the Moon was waxing, and it is mirrored for births south of the equator, where the crescent faces the other way. The methodology page describes the ephemeris and the time zone handling in more detail.

What each name means, what the Moon looked like, and how many days carried each name from January 1, 1900 to December 31, 2050 (55,152 days, counted in Universal Time). The principal phases are instants, so each names one day per cycle, 3.39% of all days; the four in-between names share the rest.

The 1,868 lunar cycles in that span averaged 29.53061 days from new moon to new moon, but single cycles ran from 29.275 to 29.83 days, because the Moon's orbit is an ellipse and the Earth's is too.

Dane Rudhyar (1895–1985), a composer and astrologer, read the Sun–Moon cycle as a model of how anything develops from a start to a culmination and a release. He called the first quarter a crisis in action and the last quarter a crisis in consciousness, and he assigned a personality type to each 45° slice. The descriptions below follow his outline in plain terms; they are interpretation, not measurement.

The slices are equal in angle but not in time. Over 1900–2050, the four types around the new and full moons (New Moon, Gibbous, Full Moon, Balsamic) lasted 3.65 days on average and took 12.4% of the time each, while the four around the quarters lasted 3.73 days and took 12.6%. The cause is the variation, a speeding-up of the Moon near new and full moon first described by Tycho Brahe. Single slices ranged from 3.17 to 4.17 days.

Steve Jobs was born at 7:15 PM Pacific Standard Time on February 24, 1955, in San Francisco, according to his birth certificate; that was 03:15 UT on February 25, 1955. The API returns:

The new moon fell on February 22, 1955, in San Francisco, two days before the birth, and the first quarter not until March 1, 1955. The birth date holds neither, so by the almanac rule it is a waxing crescent day. The U.S. Naval Observatory's table gives that new moon as 15:54 UT; the API's time is 16 seconds later, inside the table's one-minute rounding.

That evening the Sun set at 5:58 PM. At the moment of birth the crescent, 7.8% lit, stood about 16° above the western horizon, and it set at 8:41 PM. That is the textbook picture of a young waxing crescent: a thin bow in the west after dark, following the Sun down.

The Moon was 32.0° ahead of the Sun in longitude, inside the first 45° slice, so his Rudhyar type is New Moon, not Crescent. Nothing is wrong with either answer; they measure different things. Note also that the true separation, 32.35°, is a little larger than the longitude difference, because the Moon was 5.05° north of the ecliptic, close to its greatest possible latitude. The lit fraction comes from the true geometry, not from the longitude difference.

Twelve lunar cycles last about 354.4 days, 10.9 days short of a calendar year, so each birthday finds the Moon about eleven days further on in its cycle. A year after Steve Jobs's birth, his first birthday fell on a waxing gibbous Moon 98.6% lit, almost full.

The cycle comes back into step every 19 years, the Metonic cycle named after the Athenian astronomer Meton, who used it in the fifth century BC. Nineteen calendar years hold 6,939 or 6,940 days, depending on how many February 29ths they include, and 235 lunar cycles of 29.53061 days come to 6939.69 days. So the Moon on your birthday at ages 19, 38, 57 and 76 is close to the Moon you were born under. For Steve Jobs's birthday at the same clock time:

All five fall on a waxing crescent, within 12.3° of each other; the small drift comes from the leap days and from single lunar cycles running long or short. Over all 76 birthdays up to age 76, 16 fell on a waxing crescent, close to the one in five you would expect from the share of days that phase covers. To see your own birthday Moon in any year, enter that year's date with your birth time and place in the calculator above.