At full moon, we are seeing all of the moon’s day side.
And that’s what makes a full moon look full. At full moon, the moon and sun are on a line, with Earth in between. It’s as though Earth is the fulcrum of a seesaw, and the moon and sun are sitting on either end of the seesaw. So as the sun sets in the west, the full moon rises. When the sun is below our feet at midnight, the full moon is highest in the sky. When the sun rises again at dawn, the full moon is setting.
If there is a lunar eclipse, it must happen at full moon. It’s only at the full moon phase that Earth’s shadow, extending opposite the sun, can fall on the moon’s face.
Each full moon has its own name: Here are the names of all the full moons
In many ways, a full moon is the opposite of a new moon. At both the new and full phases, the moon is on a line with the Earth and sun. At new moon, the moon is in the middle position along the line. At full moon, Earth is in the middle. Full moon always comes about two weeks after new moon, when the moon is midway around in its orbit of Earth, as measured from one new moon to the next.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
Bottom line: A full moon looks full because it’s opposite Earth from the sun, showing us its fully lighted hemisphere or day side.