Moon Phases

Understand why the Moon changes shape throughout its 29.5-day cycle

The Moon does not produce its own light — it reflects sunlight. As the Moon orbits Earth (taking about 29.5 days for a complete cycle), we see different portions of its sunlit half, creating the phases. At new moon (day 0), the Moon is between Earth and the Sun, so its sunlit side faces away from us and we see darkness. Over the next ~7 days, a growing sliver appears on the right side (waxing crescent). At first quarter (day ~7.4), we see exactly half the sunlit side. The illuminated area continues to grow (waxing gibbous) until full moon (day ~14.8), when Earth is between the Sun and Moon and we see the entire sunlit face. Then the process reverses: waning gibbous, third quarter (day ~22.1), waning crescent, and back to new moon. The Moon always keeps the same face toward Earth (tidal locking). Your latitude on Earth affects the Moon's apparent orientation and path across the sky — observers in the Southern Hemisphere see the Moon 'upside down' compared to Northern Hemisphere viewers.