Moon Phase Calculator

What method does the calculator use?

It measures the time from a reference new moon, 6 January 2000 at 18:14 UTC, to 12:00 UTC on the date you enter, divides by the mean synodic month of 29.530589 days and keeps the remainder. That remainder is the number of days into the current lunar cycle. Everything else on the page, including the phase name, illumination and countdowns, is derived from that single number.

How are the eight phase names assigned?

The cycle is cut into eight equal bands of about 3.69 days, and each band is centred on its exact phase. New Moon covers the 1.84 days either side of the new moon, First Quarter is centred on day 7.38, Full Moon on day 14.77, and so on. A date a day before the exact full moon is therefore still labelled Full Moon.

How is the illumination percentage calculated?

As (1 − cos(2π × days into cycle ÷ 29.530589)) ÷ 2, rounded to a whole percent. It is a smooth curve that treats the Moon’s motion as uniform. The real Moon speeds up and slows down along its orbit, so the true illuminated fraction on a given date can differ from this value by several percentage points.

How close is it to the real new and full moons?

It follows the mean cycle, but actual new and full moons arrive earlier or later than that average by up to many hours. Because the tool also works on whole dates at noon UTC, a phase boundary can be off by a day. As one check, the real full moon of 21 January 2000 falls on the day the tool labels Full Moon. For minute-level timing use a published almanac.

What is the lunar day?

The number of days into the cycle rounded up, so the first day after the new moon is day 1 and a date 14.7 days in is day 15. Because the month is 29.53 days long, the value runs from 1 to 30, and day 30 occurs only in the last half day of a cycle.

Why is the synodic month used instead of 27.32 days?

Phases depend on the angle between the Sun and Moon as seen from Earth. The Moon returns to the same place among the stars in about 27.32 days, but Earth has moved along its orbit in that time, so the Moon needs about two more days to reach the same phase again. That makes the phase cycle 29.53 days.

Does the result depend on where I am?

No location is used. The phase is the same everywhere at the same instant, but your local date may differ from the UTC date used here. The emoji show the northern-hemisphere view, with a waxing moon lit on the right; in the southern hemisphere the lit side appears on the other side.

Does the tool say anything about moon phases and gardening or astrology?

No. It reports the phase and the day counts only. Traditions such as lunar gardening and astrological moon rituals attach meanings to the phases, but those are traditional beliefs that have not been scientifically validated, and the tool makes no claim about them.