The Moon does not make its own light
The Moon shines because it reflects sunlight. At any moment, the Sun lights up half of the Moon, just as it lights half of Earth, which is the half facing the Sun. The other half is dark. The phases we see are different amounts of that lit half turned toward us.
So the Moon never actually changes shape. What changes is the angle between the Sun, the Moon and the observer on Earth as the Moon moves along its orbit.
A walk around the orbit
The Moon travels around Earth once in roughly a month. Follow it around the circle to see how the appearance changes.
- New Moon. The Moon is roughly between Earth and the Sun. Its lit side faces away from us, and we see the dark side, so the Moon is basically invisible. It is also in the sky during the daytime, close to the Sun, so it is lost in the glare.
- Waxing crescent. A few days later, the Moon has moved along its orbit, and we see a thin sliver of the lit side, low in the western sky after sunset. “Waxing” means growing.
- First quarter. About a week after new Moon, the Moon is a quarter of the way around its orbit. We see half of the disk lit, which is half of the lit hemisphere. It is called a quarter because it is a quarter of the way through the cycle, not because a quarter of the disk is lit.
- Waxing gibbous. More than half the disk is lit, and growing.
- Full Moon. Earth is roughly between the Sun and the Moon. The side facing us is completely lit.
- Waning gibbous, last quarter and waning crescent. The lit part shrinks day by day until the Moon returns to new.
Then the cycle starts again.
How long does it take?
The cycle from one new Moon to the next takes about 29.5 days. This is a bit longer than the time the Moon takes to orbit Earth relative to the stars, which is about 27.3 days. The difference occurs because Earth is also moving around the Sun during that time, so the Moon has to travel a little farther to catch up to the same Sun-Earth-Moon arrangement.
The word “month” comes from the Moon, and many traditional calendars are based on this cycle.
It is not Earth’s shadow
A very common idea is that the phases are caused by Earth’s shadow falling on the Moon. That is not correct. Earth’s shadow falls on the side of Earth away from the Sun, so it can only reach the Moon when the Moon is on that far side, at full Moon. Even then, the shadow usually passes above or below the Moon, and it touches the Moon only during a lunar eclipse, which is rare and is covered in our explanation of how eclipses happen. Phases happen every month, and a crescent Moon is near the Sun in the sky, far from the direction of Earth’s shadow.
Waxing, waning and which side is lit
From the Northern Hemisphere, the right side of the Moon is lit during the waxing phases, and the left side during the waning phases. In the Southern Hemisphere, the Moon appears flipped, so the lit side is reversed, because observers are oriented differently. Near the equator, a crescent can look like a bowl or a boat on its back.
When the Moon rises
The phase tells you roughly when the Moon is up.
- A new Moon rises and sets with the Sun.
- A first quarter Moon is high in the sky around sunset, and sets around midnight.
- A full Moon rises near sunset and sets near sunrise, so it is up most of the night.
- A last quarter Moon rises around midnight and is high in the morning.
The Moon is not only seen at night. A first or last quarter Moon is often visible in daylight, which surprises some people.
Earthshine and the same face
Sometimes in a thin crescent phase, you can faintly see the rest of the Moon’s disk. This is called earthshine: sunlight reflecting off Earth and onto the Moon’s night side. From the Moon, Earth would be near its full phase then.
We always see the same side of the Moon, because its rotation matches its orbit period. This is called tidal locking. It does not mean one side is always dark. The far side gets as much sunlight as the near side, just at different times. During new Moon, the side we never see is the one in full sunlight.
The Moon and the tides
The Moon’s gravity helps drive the ocean tides, and tides are slightly stronger around new and full Moons, when the Sun and Moon line up. This is a separate effect from lighting, but it follows the same monthly pattern. The Sun, Earth and Moon also have a geometry that sometimes lines up exactly, creating an eclipse.
Try it at home
You can model the phases in a dark room with a single lamp and a ball. Hold the ball at arm’s length and turn slowly in a circle while the lamp stays in place. The ball represents the Moon, your head is Earth, and the lamp is the Sun. You will see the lit part of the ball change from a crescent to full and back as you turn, even though half of the ball is always lit. Observing the real Moon over a couple of weeks, noting when and where it rises, is a rewarding way to see the same pattern in the sky. The Moon also appears in the sky alongside the seasonal constellations, which shift through the year.