A Lunar Phase Calculator computes lunar phase from the inputs you provide. It applies the standard formula to the values you enter and returns the result instantly, without sending any data to a server. Free, in-browser, no signup. The tool.
29.53-day synodic cycle. Reference: Jan 6, 2000 = new moon.
Moon Phase
Lunar synodic cycle: 29.53059 days from new moon to new moon. Phases: new (0%), waxing crescent, first quarter (50%), waxing gibbous, full (100%), waning gibbous, last quarter (50%), waning crescent. Affects tides, sleep, some animals.
The Moon's phase is the fraction of its Earth-facing side that the Sun is lighting up at a given moment. As the Moon orbits Earth, the angle between the Sun, Earth, and Moon changes, so we see a steadily shifting slice of the lit hemisphere, from the invisible new moon through the brilliant full moon and back again.
This calculator finds the phase for any date by measuring how far along the current cycle the Moon has progressed since a known new moon. The cycle of phases has governed calendars, agriculture, fishing, and festivals for millennia: lunar and lunisolar calendars are built directly on it, and tides, some animal behavior, and cultural events such as Easter and Ramadan are all tied to the Moon's position. The phase is a geometric, predictable quantity, which is why it can be computed years ahead from a single reference point and the length of one cycle.
The phase is found from the Moon's age, the number of days since the last new moon, divided by the length of one synodic month. The fractional position then maps onto the eight named phases.
age = (date - reference_new_moon) mod 29.53059 (days) phase_fraction = age / 29.53059 (0 = new, 0.5 = full) illumination ~= 0.5 x (1 - cos(2 x pi x phase_fraction)) 29.53059 days = mean synodic month reference_new_moon = a known new-moon date (e.g. 2000-01-06)
Find the Moon's phase 20 days after a known new moon.
Each phase spans roughly 3.7 days of the 29.53-day cycle. Illumination values are approximate midpoints.
| Phase | Approx age (days) | Illuminated |
|---|---|---|
| New moon | 0 | 0 percent |
| Waxing crescent | 1 to 6 | 1 to 49 percent, growing |
| First quarter | ~7.4 | 50 percent, growing |
| Waxing gibbous | 8 to 14 | 51 to 99 percent, growing |
| Full moon | ~14.8 | 100 percent |
| Waning gibbous | 16 to 21 | 99 to 51 percent, shrinking |
| Last quarter | ~22.1 | 50 percent, shrinking |
| Waning crescent | 23 to 29 | 49 to 1 percent, shrinking |
One complete cycle from new moon to the next new moon is the synodic month, averaging 29.53059 days (about 29 days, 12 hours, 44 minutes). This is longer than the 27.32-day sidereal month it takes the Moon to orbit Earth relative to the stars, because Earth also moves around the Sun, so the Moon must travel a little extra to line up with the Sun again.
In order through one cycle: new moon (0 percent lit), waxing crescent, first quarter (50 percent lit and growing), waxing gibbous, full moon (100 percent lit), waning gibbous, last quarter (50 percent lit and shrinking), and waning crescent. Waxing means the illuminated fraction is increasing toward full; waning means it is decreasing toward new.
The Moon completes one orbit around Earth in 27.32 days (the sidereal month), but during that time Earth has moved about 27 degrees along its own orbit around the Sun. The Moon therefore has to travel for roughly two extra days to catch up and return to the same Sun-Earth-Moon alignment that defines a new moon, giving the 29.53-day synodic month.
Because the 29.53-day synodic month is shorter than most calendar months, a single month occasionally contains two full moons; the second is popularly called a blue moon. It happens about every 2.7 years. The name has nothing to do with color, though volcanic ash or smoke particles can rarely tint the Moon genuinely bluish.
Yes. The largest spring tides occur near new and full moon, when the Sun and Moon align and their gravitational pulls add together. The smallest neap tides occur near the first and last quarter, when the Sun and Moon pull at right angles and partially cancel. The phase therefore signals the tidal range even though tides are driven by gravity, not light.