Moon Phase Calculator

How this calculator works

The Moon takes about 29.53 days to go from one new moon to the next a period astronomers call a synodic month. This calculator counts the days between your chosen date and a known reference new moon (January 6, 2000), divides by the synodic month length, and uses the resulting fraction to determine both the current phase name and an estimated illumination percentage, calculated from the angle between the Sun, Earth, and Moon.

The eight phases, in order

New Moon (fully dark), Waxing Crescent (a growing sliver), First Quarter (half-lit, growing), Waxing Gibbous (more than half, still growing), Full Moon (fully lit), Waning Gibbous (more than half, shrinking), Last Quarter (half-lit, shrinking), and Waning Crescent (a shrinking sliver) then back to New Moon to start the cycle over. "Waxing" always means growing brighter; "waning" always means shrinking.

Why moon phase still matters

Moon phase isn't just trivia. Tides are strongest called spring tides during the new and full moon, when the Sun and Moon's gravity line up, and weakest during the quarter phases. Some gardeners still plant by phase, following older folk traditions tied to moisture cycles, though there's no verified scientific link. Photographers and stargazers often plan around the new moon specifically, since a dark sky with no moonlight makes faint stars, the Milky Way, and deep-sky objects far easier to see.

FAQ

How accurate is this?
This uses the Moon's average cycle length, which is accurate to within about a day for most purposes precise enough for everyday reference, though official astronomical almanacs account for small additional variations.

Can I check a past or future date?
Yes pick any date to see its moon phase, past or future.

What actually causes the moon's phases?
It's sunlight, not shadow a common myth is that Earth's shadow causes phases, but that only happens briefly during a lunar eclipse. Normal phases happen because we see different amounts of the Moon's sunlit half as it orbits Earth, the same way a ball lit from one side looks different depending on where you stand.

Does the moon phase look the same everywhere on Earth?
The exact phase is the same worldwide on a given date, but its orientation in the sky differs in the Southern Hemisphere, a waxing crescent appears to curve the opposite way it does in the Northern Hemisphere, because observers there are effectively looking at the sky upside down relative to northern viewers.

Why doesn't the illumination percentage exactly match the phase name?
Illumination is a continuous number from 0-100% while phase names describe eight zones along that scale, so two dates showing "Waxing Gibbous" can have noticeably different illumination percentages depending on how far into that zone they fall.