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Why does the Moon appear to change shape during a month?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
The Moon does not actually change its physical shape; rather, it appears to change shape due to its orbital geometry in relation to Earth and the Sun.1
As the Moon orbits Earth, the amount of its sunlit surface visible from our perspective changes, creating what are known as lunar phases. These varying appearances occur because:23
- New Moon: The Moon is positioned between Earth and the Sun, causing the side in shadow to face Earth.23
- Full Moon: The Moon is on the opposite side of Earth from the Sun, allowing the fully illuminated side to face Earth.23
- Quarter and Gibbous Phases: These occur at intermediate positions where the Moon is at varying angles relative to the Sun, resulting in different portions of the lunar disk appearing illuminated to an observer on Earth.23
This cycle of illumination repeats over the course of a synodic month, which lasts approximately 29.531 days.3