The Moon is not actually a full disk. It is a sphere, and only half of it is illuminated by sunlight at any given time. The half that is illuminated is what we see when we look up at the Moon from Earth.
The phases of the Moon occur as the Moon orbits the Earth. As the Moon moves around the Earth, the angle at which sunlight strikes its surface changes. This causes different amounts of the Moon's illuminated side to be visible from Earth.
When the Moon is between the Earth and the Sun, we see a full Moon. This is because the entire illuminated side of the Moon is facing us. As the Moon moves around the Earth, we start to see less of the illuminated side. This causes the Moon to go through phases, such as the crescent Moon, the gibbous Moon, and the waning Moon.
The Moon's phases are a natural occurrence that has been observed for thousands of years. They have been used to track time and to create calendars.
