What Is a Full Moon, and Why Does It Look So Big?

A full moon is the night the whole face of the moon looks lit up. It is the moment most people picture when they think of the moon. Here is what is really going on, and why it sometimes looks enormous.

What makes a moon "full"

The moon does not make its own light. It reflects the sun. A full moon happens when Earth sits between the sun and the moon, so the sunlit side of the moon is pointed almost straight at us. We see the entire lit half, which is why it looks like a complete bright circle.

This lines up about once a month, roughly every 29.5 days. You can see tonight's exact phase and how many days until the next full moon on moontrack.

Why it sometimes looks giant

A full moon low on the horizon can look huge, almost unreal. It is not actually bigger. This is called the moon illusion. When the moon is near trees, roofs, and hills, your brain compares it to those things and reads it as large. High in the sky with nothing next to it, the same moon looks normal.

A simple test: hold your thumb at arm's length next to the "giant" moon on the horizon, then check again an hour later when it is higher. Your thumb covers the same amount both times.

How bright is it, really

A full moon is bright enough to cast shadows and wash out faint stars, which is why stargazers avoid full-moon nights. If you want a dark sky for stars, aim for the days around a new moon instead.

Want the plain-English version of every phase? Read the 8 moon phases explained.