Moon Phase Calculator

The 8 moon phases explained

The moon orbits the Earth once every 27.3 days, but because the Earth is also moving around the Sun, the moon needs about 29.53 days to return to the same phase as seen from Earth. This slightly longer period is the synodic month, and it is divided into eight named phases that describe how much of the lit side we can see.

The moon sits between the Sun and Earth, so the side facing us is dark. The sky is at its best for stargazing.

A thin sliver of light appears on the right edge and grows each night. Best viewed in the western sky after sunset.

Exactly half the face is lit — the right half in the Northern Hemisphere. The moon rises around midday and sets around midnight.

More than half is lit and growing. The moon is visible for most of the evening and dominates the night sky.

The entire sunlit face is visible. The moon rises at sunset and sets at sunrise, remaining in the sky all night long.

The illuminated portion begins to shrink on the right side. Moonrise slips later into the evening each night.

Half the face is again lit — this time the left half in the Northern Hemisphere. The moon rises around midnight.

A thin crescent of light remains, visible in the eastern sky before sunrise. The cycle then resets at the next new moon.

How the calculator works

The tool computes the moon's age — the number of days since the last new moon — based on a known reference new moon and the mean synodic period of 29.530588 days. From the age it derives both the phase name and the illumination percentage. Because the method is astronomical rather than observational, it works identically for any date: a birthday in 1974, tomorrow's harvest moon, or the first full moon of the 22nd century.