Why Does the Moon Have Phases? A Plain Explanation

The Moon has phases because the Sun always lights half of it, and our view of that lit half changes as the Moon orbits Earth every 29.5 days, not Earth's shadow.

Category: Astronomy · Created: 2026-10-15 · Updated: 2026-10-15 · 3 min read

The Moon has phases because the Sun always lights one half of it, and the angle between the Sun, Earth and Moon changes as the Moon orbits Earth. From Earth we see a varying part of the lit half, from a thin crescent to a full disc. The full cycle, from one new moon to the next, takes about 29.5 days. Phases are not caused by Earth's shadow falling on the Moon.

What is lit, and what do we see?

Sunlight always strikes the side of the Moon that faces the Sun, so at any moment about half of the Moon is lit and half is in darkness. The boundary between the two halves is called the terminator. We see the Moon from Earth, so the phase depends on how much of the lit half faces us. When the Moon is between Earth and the Sun, its lit side points away and it looks dark. When it is on the far side of Earth, its lit side faces us and it looks full. As the Moon moves around Earth, the angle we view it from changes, so the boundary between light and dark sweeps across the face we see.

How does the orbit change the view?

The Moon takes about 27.3 days to complete one orbit relative to the stars. The phase cycle is longer, about 29.5 days, because Earth moves along its own orbit around the Sun during that time. The Moon has to travel a little further to line up with the Sun again. The four main phases follow this geometry:

New moon: the Moon sits roughly between Earth and the Sun, so its lit side faces away from us.
First quarter: the Moon is about 90 degrees from the Sun, and half of the Moon looks lit from here.
Full moon: the Moon is on the far side of Earth from the Sun, and its whole lit face is visible.
Last quarter: the Moon is about 90 degrees past full, on the other side, and again half looks lit.

The shapes in between are the crescents and gibbous phases. Between new and full, the lit part grows, which is the waxing phase. Between full and new, it shrinks, which is the waning phase. In the Northern Hemisphere, a waxing Moon is lit on its right side. Observers in the Southern Hemisphere see the view turned around, so the same lit side appears on the left. The same alignment that sets the phases also governs eclipses, covered in solar eclipses. A lunar eclipse can only happen at full moon, and a solar eclipse only at new moon, so every eclipse is a special case of the phase cycle.

You can check this geometry yourself by watching the Moon over several evenings. A waxing crescent sets soon after the Sun, while a full moon rises near sunset and sets near sunrise. Those times follow from the same angles that make up the phases.

Why is there not an eclipse every month?

Earth's shadow falls on the Moon only during a lunar eclipse. The Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. So at most full moons it passes just above or below Earth's shadow, and at most new moons it passes just above or below the Sun. Eclipses happen only when the alignment is close to the points where the two orbital planes cross. The way orbits are shaped and tilted is described in Kepler's laws of planetary motion.

What do people commonly get wrong?

The Moon has a dark side, but it is not permanently dark. The Moon turns once on its axis for each orbit, so the same near face always points toward Earth. This state is called tidal locking. The far side receives as much sunlight over a month as the near side, but we never see it from Earth. Phases describe only the near side, which is why the far side seems to be missing.

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