What Moon Phase Was I Born Under? How to Find It

The phase of the Moon is an angle. Seen from the Earth, the Moon sets off from the Sun’s direction at every new moon and laps it again about a month later; at any moment in between, the phase is how many degrees it has pulled ahead. That angle, the elongation, decides how much of the disc is lit and which phase name applies. So the phase you were born under comes from two numbers, the Moon’s longitude and the Sun’s at your birth minute, and the same subtraction gives the Hindu tithi.

The arithmetic is the easy part. The harder part is that “phase” is defined in more than one way, and a list of eight phase names can cut the month in two different places. This page sets out the definitions, measures how often they disagree, and works three births through.

Four Instants and Four Spans

In astronomy the four principal phases are not periods at all. They are instants: the moments when the Moon is exactly 0°, 90°, 180° and 270° ahead of the Sun. Everything between two of those instants belongs to one of the four minor phases. Wikipedia’s article on lunar phases gives the definition in that form, and it is the one the site’s moon phase calendar for 2027 uses when it prints the minute of each quarter.

On this strict reading, almost everybody was born in a crescent or a gibbous phase. Nobody is born “at” a full moon except by coincidence of the minute. The ephemeris behind this page was checked against all 58 new, first-quarter, full and last-quarter instants of November 2026 to December 2027 computed from the JPL DE421 ephemeris; the largest difference was 1.5 minutes.

Eight Phases: Two Ways to Cut the Month

A list of eight phases has to turn each instant into a stretch of time, and there are two natural ways to do it. One gives each principal phase the 45° that follow its instant, so “full moon” runs from 180° to 225°. The other centres each principal phase on its instant, so “full moon” runs from 157.5° to 202.5°. Call them the after and the centred conventions. Both have exactly the same eight names, and both give each phase an eighth of the circle. They still disagree whenever the elongation is in the second half of any 45° slice, which, sampled every three hours from 1950 to 2050, comes to 50.0 percent of the time. So an eight-phase answer means little without knowing which cut it uses; for half of all births the two give neighbouring names.

The lit share is (1 − cos e) ÷ 2, where e is the elongation. It ignores the Moon’s small tilt above or below the Sun’s path, and against the 42 lit percentages that DE421 gives at the moonrises around each full moon in the site’s full moon moonrise tables it was never more than 0.2 percentage points out. Wikipedia’s glossary of astrology lists “balsamic” as another name for the waning crescent, notes that the source of the name is unknown, and records that it was formerly called the old crescent.

Getting the Two Degrees and the Phase

The free kundli prints the Moon and the Sun for your birth minute as full longitudes from 0° to 360°, after you sign in and enter the date, time and place. Subtract the Sun from the Moon and add 360 if the answer is negative. The result is the elongation; read the phase off either table above. Sidereal or tropical makes no difference to this one angle, since the same offset is taken off both bodies.

Two more figures fall out of the same angle. The lit share follows from the formula above. The Moon’s age, the days since the last new moon, is roughly the elongation divided by 12.2, because that is about how many degrees the Moon gains on the Sun in a day; the exact age needs the new moon itself, which the examples below take from the ephemeris.

Three Births Worked Through

None of these three people exists; the times are IST, and each degree is cut to the whole arcminute, the form the free kundli uses.

17 March 1994, 07:40. The Moon reads 24°44′ and the Sun 332°29′. Subtracting leaves a negative number; adding 360 gives an elongation of 52.25°, with the disc about 19 percent lit, 4.8 days after the new moon. Every definition agrees here: waxing crescent. When the angle sits well inside a slice of both conventions, the name is not in doubt.

20 October 2001, 03:10. The elongation is 40.74° and only about 12 percent of the disc is lit. The after convention still calls this the new moon phase, because its new moon slice runs to 45°; the centred convention calls it a waxing crescent, because its new moon slice ended at 22.5°. The tithi is Shukla Chaturthi, and since a phase boundary cuts that tithi in two, the tithi alone cannot settle the after-convention name.

23 November 1988, 14:30. This birth is in the Purnima tithi, with 99.9 percent of the disc lit. The centred convention calls it a full moon. The four-span definition and the after convention both call it waxing gibbous, because the Moon has not yet reached 180°; it is 3.78° short.

Reading the Phase From a Tithi

If you already know your birth tithi, it narrows the phase down, because a tithi is a 12° step of the same angle. Of the 30 tithis, 24 lie wholly inside one slice of the after convention, as the table above lists. The other six are each cut by a slice edge: Shukla Chaturthi (45°), Shukla Ashtami (90°), Shukla Dwadashi (135°), Krishna Chaturthi (225°), Krishna Ashtami (270°), Krishna Dwadashi (315°). A birth in one of those six needs the degrees. The centred convention’s edges fall elsewhere and cut eight tithis, so under it the degrees are needed more often still. The method for finding a birth tithi from a date and time is on birth tithi from date of birth.

Two tithis catch people out. Purnima is the last 12° before the full moon, from 168° to 180°; the full moon instant is the moment that tithi ends. So anyone born in the Purnima tithi was born under a waxing Moon on the four-span and after definitions, however full it looked. Amavasya is likewise the last 12° before the new moon, so an Amavasya birth is a waning crescent, the slice some astrologers call balsamic. A panchang names a date by its sunrise tithi, though, and a Purnima date can hold the full moon instant later in the day, so a birth on that date after the instant falls in Krishna Pratipada, in the full moon slice of the after convention and in the waning gibbous span of the four-span definition. How the fortnights fall through the year is in Shukla and Krishna paksha dates.

How Long Each Phase Lasts

An eighth of the circle is not an eighth of a fixed number of days. Over the 1249 complete lunations from 1950 to 2050, a single after-convention slice lasts anywhere from 3.18 to 4.17 days, because the Moon’s speed changes with its distance from the Earth. There is also a steady pattern underneath. The four slices that begin or end at a new or full moon average less time than the four that touch a quarter:

The difference is small, at most 12.37 percent against at least 12.64 percent where an even split would be 12.5, and it matches the Moon gaining on the Sun a little faster around new and full moon than around the quarters (the periodic term astronomers call the variation; Jean Meeus’s Astronomical Algorithms, chapter 47, lists it as the 0.658° sin 2D term in the Moon’s longitude). So, measured by the clock, a birth is slightly more likely to fall in a phase that touches a quarter than in one that touches the new or full moon. That is a statement about time, not about people. The same unevenness in the Moon’s motion is why some tithis are shorter than the gap between two sunrises and others longer, as kshaya and vriddhi tithis shows.

Why the Birth Minute Matters

A table that prints one phase for each date has to pick one moment of that date. Suppose it uses midnight UTC, which is 05:30 IST. Sampled every three hours from 1950 to 2050, the after-convention phase at the actual minute differs from the phase at 05:30 IST on the same date for roughly one birth in eleven or twelve, and picking 17:30 IST instead gives much the same share. A calendar drawn up for another time zone can also put the change on a different date, which matters for Indian births looked up in a table made for London or New York. With the two degrees from your own birth minute, neither problem arises.

This page gives the geometry and nothing else. It does not read a phase as a description of the person born under it. For the Moon’s distance at each full moon, see supermoon dates and distances, and for how many full and new moons a year holds, how many Purnimas and Amavasyas in a year.

In Brief

• The phase: Moon minus Sun, plus 360 if negative; either zodiac works
• Principal phases: instants at 0°, 90°, 180°, 270°; between them, crescent or gibbous
• Eight phases: 45° after each instant, or 45° centred on it; the two disagree for half of all births
• Lit share: (1 − cos e) ÷ 2, so 15% at 45° and 85% at 135°
• From a tithi: 24 of 30 tithis give one after-convention phase; Purnima is waxing, Amavasya a waning crescent
• Length: 3.2 to 4.2 days per eighth; slightly longer near the quarters