The moon produces no light. Sunlight falls on it exactly as it falls on Earth, lighting one hemisphere and leaving the other dark. That is true every day of the month and never changes. What changes is where we are standing relative to that lit hemisphere.
When the moon is roughly between Earth and the sun, the lit side faces away from us and we see the dark side: new moon. When the moon is on the far side of Earth from the sun, the lit side faces us fully: full moon. In between we see the lit hemisphere at an angle, so it appears as a crescent, a half, or a gibbous shape.
The eight phases, in order
New Moon
The lit hemisphere faces away from us. The moon is lost in the sun’s glare and rises and sets with it.
Waxing Crescent
A thin sliver of light on the right, growing each evening after sunset.
First Quarter
Half lit on the right. Rises around noon, reaches its highest point at sunset, and sets around midnight.
Waxing Gibbous
More than half lit and still swelling toward full.
Full Moon
Fully lit, opposite the sun in the sky. It rises at sunset and sets at sunrise.
Waning Gibbous
Past full and shrinking, still mostly lit through the night.
Last Quarter
Half lit on the left. Rises around midnight, reaches its highest point at sunrise, and sets around noon.
Waning Crescent
A shrinking sliver on the left, seen low in the sky before dawn.
The shadow misconception
A very common belief is that the phases are caused by Earth’s shadow falling on the moon. They are not. Earth’s shadow does fall on the moon occasionally — that is a lunar eclipse, it happens at most a few times a year, and it looks completely different: the moon turns a dull coppery red rather than going dark.
You can disprove the shadow explanation from your own back garden. A first quarter moon is visible in the afternoon sky while the sun is still up. If Earth’s shadow were cutting the moon in half, that shadow would have to be cast sideways by a sun that is plainly in the wrong place for it.
Why the cycle is 29.5 days, not 27.3
The moon completes one orbit around Earth in 27.32 days — the sidereal month, measured against the fixed stars. But the phase cycle takes 29.53 days, the synodic month. The extra two days exist because Earth is also moving around the sun.
By the time the moon has completed one full orbit, Earth has travelled about 27° along its own path, so the sun is in a different direction. The moon has to swing a little further to line up with the sun again and complete the phase cycle. That catch-up is the difference between the two figures.
The same face, always
The moon rotates once on its axis in exactly the time it takes to orbit Earth. This is not a coincidence; it is tidal locking, the end state of billions of years of tidal friction gradually braking the moon’s spin until it matched its orbit.
The consequence is that we only ever see one hemisphere. The far side was completely unknown until the Soviet probe Luna 3 photographed it in 1959, and it turned out to look strikingly different — heavily cratered, with almost none of the dark volcanic plains that give the near side its familiar markings.