The phases of the Moon are caused by the changing positions of the Moon, Earth, and Sun as the Moon orbits the Earth. The Moon itself does not produce light; instead, we see sunlight reflected off its surface. Because only half of the Moon is illuminated by the Sun at any time, the portion of the illuminated half visible from Earth changes as the Moon moves around Earth, creating the different phases we observe
. Key points explaining the cause of the Moon phases:
- The Moon orbits the Earth approximately every 27.3 days, but the full cycle of phases (a lunar month) takes about 29.5 days due to Earth's simultaneous orbit around the Sun
- The phases range from New Moon (when the Moon is between Earth and the Sun, so the illuminated side faces away from Earth) to Full Moon (when Earth is between the Sun and the Moon, so the fully illuminated side faces Earth), with intermediate phases like waxing and waning crescents and gibbous shapes in between
- The changing angle between the Sun, Moon, and Earth determines how much of the Moon's sunlit side is visible from Earth, causing the apparent shape of the Moon to change
- The Earth's shadow does not cause the regular phases of the Moon; it only causes lunar eclipses when the Earth’s shadow falls on the Moon during a full moon
In summary, the phases of the Moon are the result of the Moon's orbit around Earth and the varying angles at which sunlight illuminates the Moon's surface as seen from Earth. The Moon's phases are essentially the changing portion of the sunlit half of the Moon visible to us. Earth's shadow only causes eclipses, not the regular phases
