Moon Phase Calculator

Enter a date to find the moon age, the moon phase (new moon, first quarter, full moon, last quarter and more), the illuminated fraction, and the days until the next full and new moon, with a diagram of the lit moon.

Input

Enter the date you want as a calendar year, month, and day. The time is optional and is treated as Coordinated Universal Time (UTC).

Year

Month

Day

days

Hour

The time is treated as Coordinated Universal Time (UTC)

Minute

min

Result

Moon age (days since the new moon)

12.4days

Waxing gibbous

Moon phase

Waxing gibbous

Illuminated fraction

93.8 percent

Until next full moon

2.4 days

Until next new moon

17.1 days

The moon is currently waxing toward full.

The Julian day of the entered date is 2,461,219.

How it works

The moon age is the number of days since the new moon. It is found from a known reference new moon at Julian day 2451550.1 (around January 6, 2000), taking the elapsed time modulo the mean synodic month of 29.530588853 days.
The illuminated fraction is approximated as (1 - cos(2π × phase)) ÷ 2, giving 0 at new moon and 1 at full moon. The phase runs from 0 to 1, with 0 and 1 being new moon and 0.5 being full moon.
The phase name divides the phase into eight parts: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent.
The date and time you enter are treated as Coordinated Universal Time (UTC). To use a local time such as your own zone, subtract the time offset before entering it.
This tool uses a mean synodic month approximation. Because it ignores the elliptical orbit of the moon, the result can differ from the true phase time by several hours up to roughly half a day.
The Julian day is computed from the calendar year, month, day, hour, and minute using a standard conversion formula.

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