The phases of the Moon are caused by the changing positions of the Moon, Earth, and Sun. The Moon does not produce its own light; instead, it is illuminated by the Sun. As the Moon orbits the Earth, the portion of the Moon's sunlit side visible from Earth changes, creating the different phases

. Because the Moon is tidally locked to Earth, the same side always faces us, but the angle at which sunlight hits the Moon changes as it moves around Earth. This changing illumination causes the Moon to appear to wax (grow brighter) from New Moon to Full Moon and then wane (grow dimmer) back to New Moon over about 29.5 days, which is the length of a lunar month or synodic period

. The main phases are New Moon (when the Moon is between Earth and Sun and the sunlit side faces away from Earth), First Quarter, Full Moon (when Earth is between Sun and Moon and the sunlit side faces Earth fully), and Last Quarter. Between these are intermediate phases like crescent and gibbous. The phases are not caused by Earth's shadow on the Moon (which only happens during lunar eclipses), but by the Moon's own shadow and the angle of sunlight illuminating it

. In summary:

• The Moon shines by reflected sunlight.
• Its orbit around Earth changes the visible illuminated portion.
• This changing illumination causes the phases.
• The cycle takes about 29.5 days to complete.

This interplay of positions explains why we see the Moon's phases from Earth