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How does lunar libration allow observers on Earth to see more than half of the Moon?
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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.
Lunar libration is an apparent or real oscillation of the Moon that allows observers on Earth to see approximately 59 percent of the lunar surface over time, rather than just the 50 percent visible at any single moment. This phenomenon occurs due to several factors:12
- Longitudinal Libration: Because the Moonâs orbit is elliptical, it moves at varying speeds according to Keplerâs second law. While the Moonâs rotation rate remains constant, its orbital speed changes, creating a back-and-forth "headshaking" motion that reveals more of the eastern or western edges.12
- Latitudinal Libration: The Moonâs axis is tilted slightly relative to its orbital plane. As it moves through its orbit, this tilt causes the Moonâs north and south poles to appear to alternate in tipping toward Earth.1
- Additional Factors: The Moon also rocks slightly to and fro, and the observer's vantage point on Earth shifts due to the planet's rotation, further contributing to the total amount of the lunar surface visible from Earth.2