Why does the Moon change shape?
The Moon does not actually change shape or grow and shrink — it is always a sphere, and the Sun always lights up exactly half of it. What changes is how much of that lit half we can see from Earth. As the Moon travels around us, the angle between the Sun, the Moon and our eyes shifts a little every night, so we see a different slice of the sunlit side. That is all a phase really is: a point of view.
The eight phases in order
The cycle runs New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent, then it starts again. "Waxing" means the lit part is growing, "waning" means it is shrinking, and "gibbous" describes the bulging shape between a half Moon and a full one. A quarter Moon looks like a half circle, which confuses many people: the name refers to the Moon being a quarter of the way through its orbit, not to how much of it we see.
How long is a lunar month?
One full cycle from New Moon to the next takes about 29.5 days, which is why this tool walks through roughly 30 nights. The Moon actually completes its orbit around Earth in about 27.3 days, but in that time the Earth has also moved along its own orbit around the Sun, so the Moon needs a couple of extra days to line up the same way again. That 29.5-day rhythm is where the idea of the month comes from in almost every calendar in history.
A note on hemispheres
In the Northern Hemisphere the Moon lights up from the right as it waxes and empties from the right as it wanes, which is the convention used in this widget. Seen from the Southern Hemisphere the whole thing appears flipped, so a waxing crescent points the other way. The phase is the same everywhere on Earth; only its orientation in the sky differs.
Frequently asked questions
What are the 8 phases of the Moon?
New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent. The cycle then repeats, taking about 29.5 days from one New Moon to the next.
Why does the Moon change shape?
It does not. The Sun always lights half the Moon; as the Moon orbits Earth we see that lit half from different angles, so a different amount of it is visible each night. The changing shape is a matter of perspective, not of the Moon itself changing.
How long does it take the Moon to go through all its phases?
About 29.5 days, called a synodic month. The Moon orbits Earth in roughly 27.3 days, but Earth also moves around the Sun during that time, so a couple of extra days are needed for the Sun, Earth and Moon to line up the same way again.
What is the difference between waxing and waning?
Waxing means the lit portion is getting bigger, from New Moon toward Full Moon. Waning means it is getting smaller, from Full Moon back toward New Moon.
Why is a quarter Moon shaped like a half circle?
Because the name refers to the orbit, not the shape. At First Quarter the Moon has completed a quarter of its journey around Earth, and from where we stand exactly half of its visible face is lit.