What Are the Phases of the Moon Caused By?

The Moon’s appearance in our night sky changes in a predictable cycle, transforming from a thin sliver to a full, luminous disk and back again. These regular changes are known as lunar phases, and they repeat approximately every 29.5 days. The distinct shapes we observe are a result of the interplay between the Sun, Earth, and Moon.

Common Misunderstandings

A frequent misconception about lunar phases is that they are caused by Earth’s shadow falling on the Moon. This idea is incorrect because Earth’s shadow is responsible for a distinct phenomenon known as a lunar eclipse. Lunar eclipses occur only when Earth passes directly between the Sun and a full Moon, temporarily blocking sunlight from reaching the Moon’s surface. This is a relatively rare event, happening only a few times a year, while moon phases are a continuous monthly cycle.

The Moon, like Earth, always has a side illuminated by the Sun and a side experiencing darkness. Therefore, the varying shapes we see are not due to Earth’s shadow obscuring parts of the Moon, but rather our changing view of the sunlit portion. Other misconceptions, such as clouds hiding the Moon or the Moon producing its own light, do not explain the phases.

The Earth-Moon-Sun Relationship

The true cause of the Moon’s phases lies in the dynamic orbital relationship between the Moon, Earth, and Sun. The Moon continuously orbits Earth, taking approximately 29.5 days to complete a full cycle of phases from our perspective. The Moon’s orbit is tidally locked, meaning the same side of the Moon always faces Earth.

The Moon does not generate its own light; instead, it reflects sunlight. Similar to Earth, half of the Moon’s spherical surface is always illuminated by the Sun. The appearance of the Moon from Earth depends entirely on how much of this sunlit half is visible to us as the Moon travels through its orbit.

The Moon’s appearance changes through distinct phases as it orbits Earth:

  • When the Moon is positioned between the Earth and the Sun, the side facing Earth is in darkness, resulting in the “New Moon” phase, which is largely invisible from Earth.
  • A small sliver of its illuminated side then becomes visible, appearing as a “Waxing Crescent”.
  • Approximately one week into the cycle, half of its visible face is illuminated, marking the “First Quarter” phase.
  • The illuminated portion continues to grow, leading to the “Waxing Gibbous” phase, where more than half of the Moon appears lit.
  • When the Moon is on the opposite side of Earth from the Sun, its entire sunlit face is visible, resulting in a “Full Moon”.
  • After the Full Moon, the illuminated area begins to decrease, entering the “Waning Gibbous” phase.
  • This is followed by the “Third Quarter” (or “Last Quarter”), where the other half of the Moon’s face is illuminated.
  • Finally, the Moon becomes a “Waning Crescent” before returning to the New Moon, completing the cycle.