Moon phase (also called Lunar phase) is a visible shape of the moon, as seen by observer in the sky.
The Moon is round, and it is illuminated by the Sun from only one side. If only this illuminated part (that is much brighter) is visible, the Moon has C - like or D - like shape. The changes of this shape repeat with 29.53 day period (Lunar month). This period that is longer than Moon's travel time around the Earth (27.32, the sideral month). The Moon phase depends on how the Mon, Earth and Sun are oriented with relation to each other, and is influenced by both Moon and Earth travelling around the Sun.
When the Moon is growing (waxing), the shape is concave from the left side, and when is shrinking (wanning) - from the right side. If the language allows, children are often taught to associate the shape with the similar letter, frequently adding imaginary vertical dash. For instance, in Lithuanian, the words are Dyla (shrinking) and Pilnėja (growing). We are lucky to have the impressive views of the comparatively very large satellite for the planet of our size.
The dark side of the Moon is also sometimes visible, as it is illuminated by the glow of the Earth. The shadow of the Earth has no influence on the regular Lunar phases. This shadow only touches the Moon during lunar eclipses that are infrequent events.
Lunar phase is important in astrology. It also has been subject of serious scientific studies, suspecting that it may influence the success, but the researchers found no scientific evidence
The moon phase calculation code that runs inside this applet is also available at F-Droid wrapped around an Android app.
Moon applet by Akkana Peck.