Moon phase calendar
See the moon phase and illuminated fraction for every day of a month, and the exact time of each new moon, first quarter, full moon and last quarter.
Runs entirely in your browser
Principal phases
Phases are for the middle of each day (12:00) in the time zone you choose; the exact times are accurate to a few minutes. The moon symbols show the northern hemisphere view; south of the equator the lit side is reversed. Not for navigation.
How to read the moon phase calendar
- Choose the month and the year, and the time zone to show the dates and times in.
- Read the calendar: each day shows the moon symbol and how much of the disc is lit. Days with a principal phase are highlighted.
- Look under the calendar for the exact date and time of each principal phase in the month.
How the calendar is calculated
For each day the page computes the Moon's and Sun's ecliptic longitudes and distances at 12:00 in the chosen time zone, using the series in Jean Meeus, Astronomical Algorithms (2nd ed., 1998): the Sun from chapter 25 and the Moon from chapter 47. The elongation is the Moon's longitude minus the Sun's, 0° at new moon, 90° at first quarter, 180° at full moon and 270° at last quarter. The phase name comes from the elongation in 45° steps.
The illuminated fraction is k = (1 + cos i) / 2, where i is the phase angle found from the Moon-Sun-Earth triangle (Meeus, chapter 48, formula 48.3). The exact times of the principal phases are found by scanning the elongation every six hours and bisecting each crossing of 0°, 90°, 180° and 270° to about a second. The synodic month used for reference is 29.530588853 days.
The same engine is behind the sun and moon API, which is tested against the JPL DE421 ephemeris: illumination is within 0.5 percentage points. The result is an astronomical calculation, not for navigation, and the visible shape also depends on libration and your latitude.
Questions
How long is a lunar month?
The time from one new moon to the next is the synodic month, 29.53 days on average. It varies by about half a day either way because the Moon's orbit is elliptical, so the gap between two full moons is 29 or 30 days when counted in whole days.
What is the difference between illumination and phase?
The phase is the name for the Moon's stage (new, waxing crescent, first quarter and so on), based on the angle between the Moon and the Sun. The illumination is the percentage of the visible disc that is lit. At first or last quarter the Moon is 50% lit, even though it is called a quarter.
How exact are the phase times?
The times come from the position of the Moon and Sun calculated with Jean Meeus's formulas and are within a few minutes of the values in almanacs. A phase listed for your time zone may fall on a different date from a table in UTC.
Why is the Moon drawn differently south of the equator?
Seen from the southern hemisphere the Moon appears upside down compared with the northern hemisphere, so a waxing crescent is lit on the left instead of the right. The symbols here follow the northern view.
Can I look at a past or future month?
Yes. The calculation works for any year the browser can handle. Accuracy slowly worsens far from the present because the formulas are truncated, but remains within hours for several centuries around today.
Is a supermoon shown?
No. The calendar shows phase and illumination only. The Moon's distance for the day is available in the sunrise and moon calculator.