What moon phase was I born under? Moon phase on your birthday

What moon phase was I born under?

The moon phase you were born under is the phase the moon was in at the moment of your birth, and it is one of eight: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter or waning crescent. To find it you need your date of birth, ideally the hour and time zone as well, and the time of the nearest new moon. The calculator above does this arithmetic and shows the phase name, the illuminated share of the disc, the moon's age in days and the lunar day number for your birth date.

Since the four principal phases (new moon, first quarter, full moon and last quarter) are instants rather than whole days, a birth date that lands close to one of them is a borderline case. If you were born on the calendar day of a full moon, you were born under a full moon by any reasonable definition, even if the exact moment fell a few hours before or after your birth. If the moon was 45 percent lit and waxing, you were born under a waxing crescent, about a day short of first quarter.

What are the eight phases of the moon in order?

The eight phases of the moon in order are new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent, after which the cycle begins again with the next new moon. The whole sequence takes one synodic month, about 29.5 days.

Four of these are principal phases and four are intermediate. The principal phases happen at a precise instant defined by the geometry of the sun, Earth and moon: new moon when the moon and the sun share the same ecliptic longitude, full moon when they are 180 degrees apart, and the quarters at 90 and 270 degrees. The intermediate phases, the two crescents and the two gibbous phases, are the roughly week-long periods between those instants.

The ages in the table are averages. The moon travels along an elliptical orbit at a varying speed, so the interval between two principal phases can differ from the average of 7.38 days by many hours.

What do moon illumination and moon age mean on my birthday?

Moon illumination is the percentage of the moon's visible disc that was lit by the sun on your birth date, and moon age is the number of days that had passed since the previous new moon; both are computed for 12:00 UTC on the date of birth. Illumination runs from 0 percent at new moon to 100 percent at full moon and back to 0. At first quarter and last quarter exactly half of the disc is lit, which is why those phases are called quarters even though the moon looks like a half: the word refers to a quarter of the orbit completed since new moon.

Moon age grows by one day per day, from 0 at new moon to about 29.5 at the next new moon. An age of 7.4 days corresponds to first quarter, 14.8 to full moon and 22.1 to last quarter. Near new moon and full moon the illumination changes slowly from one day to the next; near the quarters it changes by about 10 percentage points a day, so the exact hour of birth matters most for people born close to a quarter. Seen from the Northern Hemisphere a waxing moon is lit on the right and a waning moon on the left; from the Southern Hemisphere it is the other way round.

How long is a lunar month from new moon to new moon?

A lunar month from one new moon to the next, called a synodic month, lasts 29.530589 days on average, which is 29 days, 12 hours, 44 minutes and about 3 seconds. Individual months vary from roughly 29.18 to 29.93 days, because the moon's orbit is elliptical and because the Earth itself moves around the sun at a changing speed over the year.

The synodic month is longer than the sidereal month of about 27.32 days, the time the moon needs for one orbit relative to the stars, because the Earth advances along its own orbit meanwhile and the moon must travel a little more than a full circle before it lines up with the sun again.

Twelve synodic months add up to 354.37 days, about 10.9 days short of an average calendar year of 365.2425 days. That gap is the reason the phase on your birthday shifts every year, and it is the same gap that lunar calendars such as the Hebrew and the Chinese calendar close by inserting an extra month every two or three years.

Does the moon phase on my birthday depend on the time zone and hour of birth?

Yes, a little. The moon's phase is a function of the moment in time, so the same calendar date can show slightly different illumination depending on the hour and the time zone. Over one day the illumination changes by up to about 10 percentage points near the quarters and by less than one point near new and full moon, so for most birth dates the phase name is the same at any hour.

The calculation is made for 12:00 UTC on the birth date, the middle of the day in universal time. The case that needs care is a birth within a few hours of a principal phase. A full moon at 23:40 UTC on 5 March is still 5 March in London and New York, and already the morning of 6 March in Tokyo, so a Japanese almanac lists that full moon under 6 March and a British one under 5 March. If your birth falls on the day before or after a principal phase and you know your time of birth, convert it to UTC and compare it with the published time of the phase.

When will the moon phase on my birthday be the same as the day I was born?

The moon phase on your birthday returns to the phase of your birth about every 19 years, so at 19, 38, 57, 76 and 95 the moon on your birthday is within a day of where it was when you were born. In the years between, the phase drifts through the whole cycle.

The arithmetic is simple. An average calendar year of 365.2425 days is 12.368 synodic months. The twelve whole months bring the moon back to the same phase; the leftover 0.368 of a month, about 10.9 days, is the amount by which the moon's age on your birthday advances each year. After two years the moon is about 21.8 days older, after three years it has gone round once and is 3.1 days older than at your birth, and so on.

Because 19 × 0.368 = 6.997, nineteen years bring the phase back almost exactly: on average the moon on your 19th birthday is only about two hours short of the age it had at your birth. Eight years and eleven years also give near repeats, each about a day and a half off, which is why the same phase feels familiar at 8, 11, 19, 27, 30 and 38.

Real calendars are a little rougher than the averages. A span of 19 years contains four or five leap days, so it is 6,939 or 6,940 days long, while 235 lunar months are 6,939.69 days. The actual match on your 19th birthday is therefore within about 17 hours either way, which is the ±1 day of the Metonic cycle.

What is the Metonic cycle and why is it 19 years?

The Metonic cycle is the near equality of 19 tropical years and 235 synodic months: 19 years are 6,939.60 days and 235 lunar months are 6,939.69 days, a difference of about two hours. Because of it, the phases of the moon fall on the same calendar dates every 19 years, give or take a day.

The cycle is named after Meton of Athens, who described it around 432 BC and used it to bring the lunar months of the Greek calendar into step with the solar year. Babylonian astronomers knew the same relationship and used it to schedule their leap months from the 5th century BC. In 330 BC Callippus refined it into a 76-year cycle, four Metonic cycles minus one day.

Two calendars still in daily use are built on the Metonic cycle. The Hebrew calendar adds a thirteenth month in 7 of every 19 years so that its lunar months stay aligned with the seasons. The Christian calculation of the date of Easter, the computus, gives each year a golden number from 1 to 19, its position within the Metonic cycle, and uses it to find the ecclesiastical full moon after which Easter Sunday follows. The golden number is the remainder of the year divided by 19, plus one; for 2026 it is 13.

What is a lunar day and which lunar day was I born on?

A lunar day is the number of a day counted from the new moon, from 1 to 29 or 30, and the lunar day of your birth is the position of your birth date within the lunar month that was running at the time. In the simplest count, the lunar day is the whole part of the moon's age plus one, so an age of 5.8 days falls on lunar day 6 and an age of 14.8 days on lunar day 15.

Lunar days are a feature of the folk and almanac calendars of Russia and other Eastern European countries, where they are called lunnye sutki. In the traditional version of that calendar the first lunar day begins at the moment of the new moon and every following day begins at moonrise, so the first day may last only a few minutes and a month has 29 or 30 lunar days. Because moonrise occurs at a different clock time in every location, traditional calendars can disagree by one day with each other and with a count based on the moon's age alone.

Almanacs in that tradition attach meanings to each lunar day. Those meanings belong to astrology and are outside the scope of this page; here the lunar day is only a number that tells you how far into the lunar month your birth fell.

Are more babies born on a full moon?

No. The largest study on the question, published by Jill M. Arliss and colleagues in the American Journal of Obstetrics and Gynecology in 2005, examined 564,039 births in North Carolina between 1997 and 2001 and found no relationship between the phase of the moon and the number of births, the number of spontaneous deliveries or the rate of delivery complications.

The belief that maternity wards fill up at full moon is widespread, and it is easy to see how it persists: a busy night on a full moon is memorable, a busy night on a waning crescent is not. Counting every night over five years removes that bias, and the count shows a flat line across the 29.5-day cycle. Roughly one person in thirty was born on the calendar day of a full moon, the same share as for any other day of the lunar cycle.

What are the names of the full moons and where do they come from?

The popular full moon names, wolf moon, snow moon, harvest moon and the rest, come from North American tradition and were spread worldwide by the Old Farmer's Almanac, which drew on names used by Algonquian-speaking peoples and by European settlers. They are attached to the month in which the full moon falls, so if you were born under a full moon in January you were born under a wolf moon.

The harvest moon is the full moon closest to the September equinox, so in some years it falls in early October; the September full moon is then called the corn moon. A blue moon in today's usage is the second full moon within one calendar month, which happens roughly every two to three years; an older almanac definition applied the name to the third full moon in a season that has four.

Europe has its own moon folklore. In Lithuanian folk tradition the young, waxing moon was associated with growth and beginnings and the old, waning moon with endings and finishing work, so sowing or starting a task was timed by the phase. These are folk beliefs recorded by ethnographers, and they describe how people lived with the lunar calendar.

How do I find the moon phase on a date without a calculator?

To find the moon phase on any date by hand, count the days from a known new moon to your date, divide by 29.53 and keep the remainder: that remainder is the moon's age, and the table of eight phases turns it into a phase name. The method gives the phase name reliably and the age to within about a day, because individual lunar months differ from the average by several hours.

A convenient reference is the new moon of 6 January 2000 at 18:14 UTC, widely used as an epoch in astronomical formulas. Take 4 July 2015 at noon UTC as an example. From the reference new moon to that moment is 5,657.7 days. Dividing by 29.530589 gives 191.59, so 191 whole months have passed and the remainder is 17.4 days: a waning gibbous about 92 percent lit. The almanac confirms it: the full moon fell on 2 July 2015, two and a half days earlier.

• Count the days from 6 January 2000, 18:14 UTC to your date and hour, counting backwards for earlier dates.
• Divide by 29.530589, keep the fractional part and multiply it by 29.53 to get the moon's age; for a negative result add 29.53.
• Read the phase from the moon age: 0 is new, about 7.4 is first quarter, about 14.8 is full, about 22.1 is last quarter.

The phase tables published by the U.S. Naval Observatory give the exact times of the principal phases for any year, and they are the best check for a birth that falls close to a new moon, a full moon or a quarter.