Phases of the Moon
This simulation illustrates why the Moon appears as it does at different times in its phase cycle. The lower portion of the diagram shows the relative positions of the Sun, the Earth, and the Moon as seen from above the plane in which the Moon orbits the Earth. This is roughly the vantage point looking down on the Earth from above its north pole. The sunlit sides of Earth and Moon are shown as half circles; the "missing" halves are in night. Sizes and distances are not drawn to scale.
At the top is a view of the Moon as it appears from Earth at any given point in the Moon's orbit.