Moon Phase Calculator
Pick a date and time to see the Moon's phase, how much of its disc is lit, its age in days, and when the next new and full moon fall — drawn and computed live in your browser.
Runs entirely in your browser. Nothing you enter is uploaded or logged — the lunar maths uses a small committed library and works offline. Phase dates are shown on your device's local clock.
Time of day matters: the Moon moves about 12° a day, so illumination and phase shift measurably within a single day. Location is optional — the phase is the same worldwide; a southern latitude only mirrors the drawn disc.
What this tool shows
For any instant you choose — a date and a time — it reports the Moon's phase (one of the eight canonical names), the illuminated fraction of its disc as a percentage, the Moon's age in days since the last new moon, its Brown lunation number, and the dates of the next new moon and next full moon. It also draws the phase as a shaded disc. Every figure recomputes the moment you change the date or time, so you can step forward day by day and watch the crescent fill and empty.
Phase, elongation and illumination
The single number that drives everything is the elongation — the angle between the Sun and the Moon as seen from Earth. At 0° the Moon sits in the same direction as the Sun and its near side is dark: new moon. At 90° it is a quarter of the way around, half lit: first quarter. At 180° it is opposite the Sun, fully lit: full moon. At 270° it is half lit again on the other side: last quarter. The illuminated fraction follows from the phase angle by the formula k = (1 + cos i) / 2, so it does not rise evenly: it barely moves near new and full moon and changes fastest around the quarters. That is why a "waxing gibbous" looks nearly full for several nights while a thin crescent thickens quickly.
The synodic month
The phase cycle repeats every synodic month — on average 29.53 days from one new moon to the next. This is noticeably longer than the 27.32-day sidereal month, the time the Moon takes to return to the same position against the background stars. The gap exists because the Earth is itself moving: during the ~27 days the Moon takes to loop its orbit, the Earth has carried it about 27° further around the Sun, so the Moon must travel roughly two extra days' worth of arc to line back up with the Sun and complete a phase cycle. The Moon's age reported here is simply how far it is into the current synodic cycle, expressed in days.
Worked example: the new moon of 6 January 2000
Set the date and time to 6 January 2000, 18:14 UTC — a well-known reference new moon that astronomers use as an anchor for the lunation count. The tool shows:
The Brown lunation number counts synodic months from the new moon of 17 January 1923, which was assigned number 1; this reference new moon is number 953. Notice the next full moon lands about 14.8 days after new — half a synodic month, exactly as expected. Step the clock forward a week and you will watch the illumination climb through the waxing crescent toward first quarter.
Phase and when the Moon is up
Because phase measures the Moon's angle from the Sun, it also sets a rough timetable for when the Moon is visible. A new moon rises and sets with the Sun and is lost in the glare. A first-quarter moon rises around midday and sets near midnight, so it dominates the evening sky. A full moon rises at sunset and sets at sunrise, up all night long. A last-quarter moon rises near midnight and is highest at dawn. This is why a full moon feels like an all-night event and a crescent is only ever a brief companion to dusk or dawn.
| Phase | Illumination | Rises roughly |
|---|---|---|
| New | 0% | with the Sun |
| First quarter | ~50%, waxing | around midday |
| Full | 100% | around sunset |
| Last quarter | ~50%, waning | around midnight |
Honest limits
This is a simplified, geocentric model, and it is worth being clear about what that means. The phase angle and elongation are accurate to within roughly half a degree for dates within a couple of centuries of the year 2000; the tool uses only the leading periodic terms of Meeus' series and omits the smaller ones and planetary perturbations. The illuminated fraction is good to about 1%, and a little worse in the thin crescent where a small angle error matters most. The Moon's age is derived from the mean synodic month, so it can drift up to about half a day from a true almanac age, because the Moon speeds up near perigee and slows near apogee rather than moving evenly. The next new and full moon dates are found by searching that same elongation curve and are typically right to within a few minutes. Because the phase is geocentric it is the same for everyone on Earth at a given instant; only the orientation differs — from the Southern Hemisphere the lit side is mirrored, which is why entering a southern latitude flips the drawn disc but leaves every number unchanged. Finally, this is a phase calculator, not a rise/set or eclipse predictor: it does not compute moonrise, moonset, the Moon's altitude, or whether a given full moon is a total lunar eclipse — those need the Moon's declination and your exact horizon, which are outside this tool.