* Phases of the Moon: The Moon doesn't emit its own light; we see it because sunlight reflects off its surface. As the Moon orbits Earth, the amount of illuminated surface we see changes, causing the different phases:
* New Moon: The Moon is between the Sun and Earth, so we see the unlit side.
* Waxing Crescent: A sliver of the illuminated side becomes visible.
* First Quarter: We see half the Moon illuminated.
* Waxing Gibbous: More than half the Moon is illuminated.
* Full Moon: The entire illuminated side faces Earth.
* Waning Gibbous: The illuminated side begins to shrink.
* Last Quarter: Again, we see half the Moon illuminated.
* Waning Crescent: The illuminated sliver continues to shrink until it's completely dark again (New Moon).
* Daily Change: The Moon moves about 13 degrees each day in its orbit. This movement causes the amount of illuminated surface we see to change gradually, resulting in the noticeable daily shape changes.
* Timing: A complete cycle of lunar phases (from New Moon to New Moon) takes about 29.5 days, so the Moon's shape changes noticeably every few days.
In Summary: The changing shape of the Moon, or its phases, are a result of the angle at which sunlight hits the Moon as it orbits Earth. This results in a gradual change in the visible illuminated portion of the Moon's surface over the course of roughly a month.
