Phases of the Moon
The eight recurring shapes of the moon, caused simply by the changing angle from which we see its sunlit half.
Also known as: Lunar phases, New moon, Full moon
The moon's phases are the different apparent shapes it shows over roughly 29.5 days. They are not caused by any shadow of the Earth in normal months, but by the changing angle between the Sun, Earth, and Moon — we simply see varying amounts of the moon's permanently sunlit half.
What it is
The phases of the moon are one of the most familiar sights in the sky, and understanding them factually is the foundation for everything else on this site — including the lunar-gardening tradition.
What actually causes phases. The Sun always lights up half of the moon. As the moon orbits the Earth, the angle from which we view that lit half changes, so we see different fractions of it illuminated. Phases are a matter of geometry and viewpoint — not, in an ordinary month, the Earth's shadow. (The Earth's shadow only falls on the moon during a lunar eclipse, a separate and occasional event.)
The eight phases. Astronomers name eight, in order: new moon (the lit side faces away, so it appears dark), waxing crescent, first quarter (half lit, growing), waxing gibbous, full moon (fully lit, opposite the Sun), waning gibbous, last quarter (half lit, shrinking), and waning crescent, before returning to new. "Waxing" means the lit portion is growing; "waning" means it is shrinking.
The lunar month. One complete cycle from new moon to new moon — the synodic month — averages about 29.5 days. This is slightly longer than the moon's orbit around the Earth because the Earth is also moving around the Sun, so the moon must travel a little farther to return to the same phase.
Illumination. The percentage of the moon's visible disc that is lit rises from 0% at new moon to 100% at full and back again. This figure is what lunar calendars and the phase widgets on this site report, and it is fully deterministic — it can be computed precisely from astronomy, with no prediction or forecasting involved.
Grasping this geometry makes clear why moon phases are a reliable natural clock, while also making clear that the phases themselves are simply changing light, not a force acting on the world.
Worked example
Someone notices the moon looks like a growing half-circle in the evening sky and wants to name the phase. Because more of it is lit each night, it is waxing; because it is about half lit, it is the first quarter. From here the shape will keep filling toward full over the following week — pure geometry, predictable in advance because it depends only on the Sun–Earth–Moon angle.
Related entries
Sources & further reading
- Moon Phases — NASA Science (article)
- The phases of the Moon and the lunar cycle — Royal Museums Greenwich (article)