What phase is the moon in?

How it works

The Moon does not make its own light: it is always half lit by the Sun. What changes is how much of the lit half we can see from Earth. Once about every 29.5 days the Moon goes through all its phases: new moon, when the lit side is turned away from us, then a growing crescent, first quarter, a growing gibbous Moon, full moon, and back down through last quarter to a thin crescent.

The page finds the angle at the Moon between the Sun and the Earth, the phase angle i, from the positions of the Moon and the Sun, and shows the lit share k = (1 + cos i) / 2. At new moon k is 0, at the quarters 50 % and at full moon 100 %. Age is the days since the last new moon. Example: 12 days after a new moon the Moon is gibbous and about 90 % lit. The method is from Jean Meeus, Astronomical Algorithms, chapters 48 and 49. For a place, moonrise and moonset are the moments the top of the Moon touches the horizon, allowing for its nearness to the Earth (parallax) and for the air bending its light (Meeus, chapters 15 and 47).

Phase times are good to a few minutes and are the same everywhere on Earth; they are shown in UTC. The picture shows the Moon as seen from the northern hemisphere: in the southern hemisphere the lit side is the other way round. Moonrise and moonset are good to about a minute, without hills, buildings or weather; about once a month a date has no moonrise or no moonset, and near the poles the Moon can stay up or down all day. The page does not show the Moon's tilt or craters.