Free Moon Phase Calculator for Any Date

The Moon's Phase, Illumination, and Age for Any Date

A moon phase calculator tells you how much of the Moon's face is lit on a chosen date, names the phase, and shows how many days into the lunar cycle you are. Photographers planning a shot of a thin crescent, gardeners who sow by the Moon, anglers reading tides, teachers explaining the lunar cycle, and anyone curious about the Moon on their birthday all want the same thing: a clear picture and a number, without installing an app. This tool draws the lit portion of the disc, reports the illuminated percentage and moon age, and lists the upcoming principal phases, all computed in your browser. The calculation is the mean synodic model. It counts the days elapsed since a known new moon at the epoch of 6 January 2000, 18:14 UTC, and divides by the synodic month of 29.530588853 days, keeping the fractional part as the phase from 0 to 1. A phase of 0 is new, 0.25 is first quarter, 0.5 is full, and 0.75 is last quarter. The illuminated fraction follows directly from the phase as one minus the cosine of twice pi times the phase, all divided by two, which rises smoothly from 0 at new moon to 1 at full moon and back. Moon age in days is simply the phase multiplied by the synodic month, and the next new, first quarter, full, and last quarter dates come from stepping the phase forward to each target value. The model was checked against known moon events from timeanddate.com and the US Naval Observatory before shipping. At the full moon of 25 January 2024 the calculator returns a phase of 0.49 and 99.9 percent illumination; the full moon of 15 December 2024 gives 0.48 and 99.6 percent. At the new moons of 11 January and 1 December 2024 it returns essentially 0 percent illumination, and the phase reads within a hundredth of 0 as it should at a true new moon. Those results sit within the model's honest tolerance: because it uses a constant average month length and ignores the Moon's slightly elliptical orbit, the exact instant of a phase can differ from the true value by up to a few hours, though the visible shape on any given day, which is what most people actually want to see, is reliable to the eye. A night photographer uses the illumination percentage to decide whether the Milky Way will be washed out or whether a bright full moon suits a landscape. A gardener following lunar planting checks whether the Moon is waxing or waning before sowing. A teacher pulls up the month calendar strip to show a class how the phase marches from new to full and back over roughly four weeks. Someone planning a camping trip picks a weekend near the new moon for the darkest skies. The drawn disc makes the difference between a waxing and waning crescent obvious, which a percentage alone cannot convey. The key honesty note is that this is an approximation, not an ephemeris. The mean synodic method is excellent for the phase name, the illuminated fraction, and a visual, but it is not built for the exact minute of a full moon or for eclipse prediction, where the Moon's orbital variations matter; for those, consult timeanddate.com or the Naval Observatory. The phase depends only on the moment in UTC, so a date near a phase boundary can read slightly differently depending on the time of day. The disc is drawn on a canvas after the page loads, so the numbers appear even where drawing is unavailable. Everything runs locally in your browser and no date you enter is ever uploaded or stored.

Moon Phase Calculator Formula & Method

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