Moon Phase Calculator
Calculate the Moon phase for any date.
Find moon age in days, phase name, and illumination percentage.
Based on the synodic month of 29.53 days.
The lunar cycle (synodic month) lasts 29.53059 days — the time for the Moon to return to the same phase.
This is slightly longer than the Moon’s orbital period (27.32 days, the sidereal month) because Earth is also moving around the Sun, so the Moon needs a little extra time to reach the same alignment.
The 8 moon phases:
- New Moon (0–1.85 days): Moon is between Earth and Sun, invisible
- Waxing Crescent (1.85–7.38 days): First sliver appears in the west after sunset
- First Quarter (7.38–11.07 days): Half the Moon is lit (right side in Northern Hemisphere)
- Waxing Gibbous (11.07–14.77 days): More than half lit, growing toward full
- Full Moon (14.77–16.62 days): Fully illuminated, rises at sunset
- Waning Gibbous (16.62–22.15 days): Starts decreasing after full
- Last Quarter (22.15–25.84 days): Half lit again (left side in Northern Hemisphere)
- Waning Crescent (25.84–29.53 days): Slim crescent visible before dawn
Illumination formula (approximate):
illumination = (1 - cos(2π × age / 29.53)) / 2 × 100%
Reference point used: A known New Moon occurred at January 6, 2000 at 18:14 UTC (JD 2451550.26). From this, we calculate the age of the current Moon by counting elapsed days and finding the remainder when divided by 29.53059.
Historical note: The word “month” comes from “Moon.” Many ancient calendars were purely lunar. The Islamic calendar is still a purely lunar calendar. The Hebrew calendar is a lunisolar calendar that adds leap months to stay aligned with the solar year.