We see different phases of the moon because the Moon orbits the Earth and the Sun illuminates different portions of the Moon's surface that face Earth. The Moon itself does not emit light; it reflects sunlight. As the Moon moves around the Earth during its roughly 29.5-day cycle, the angle between the Earth, Moon, and Sun changes, altering the visible illuminated part of the Moon from our perspective. This causes the familiar phases of the Moon, including the new moon, crescent, first quarter, gibbous, full moon, and the waning phases in reverse order. The phases repeat as the Moon completes its orbit, making the illuminated portion appear to grow (waxing) and then shrink (waning).

Explanation of Moon Phases

• The Moon is always half illuminated by the Sun, but we only see the portion lit up that faces Earth.
• When the Moon is between the Earth and the Sun, the side facing Earth is dark (new moon).
• When the Earth is between the Sun and the Moon, the side facing Earth is fully lit (full moon).
• At intermediate points in the Moon's orbit, we see partially illuminated crescents or gibbous shapes.
• The phases cycle takes about 29.5 days, which is a lunar month or synodic month.

Why the Moon's Appearance Changes

• The changing view results from our changing vantage point on Earth relative to the Moon and the Sun.
• The Moon is tidally locked to Earth, so the same side always faces Earth, but that side’s illumination changes.
• Waxing means the illuminated part grows; waning means it shrinks.
• The phases are a natural consequence of the orbital mechanics and relative positions of Earth, Moon, and Sun.