Have you ever wondered whether the Moon rotates and why we only see on side of the Moon? The Moon is tidally locked to the Earth. What this means is it keeps the same face or hemisphere toward the Earth as it rotates around its axis. The Moon completes one rotation (a Lunar day) every 27.322 days. So, while the Moon rotates, it doesn’t appear to spin when viewed from Earth. This is called synchronous orbit. If the Moon did not spin, we’d see the entire Moon over the course of its orbit.
How Long Is a Lunar Day?
The Moon completes an orbit around Earth about once every 27.3 days. This is the length of the lunar day and also the lunar year, since a year is the time to complete an orbit around a body. Because the Earth is also moving around the Sun, it takes a bit longer for the Moon to show the same phase from Earth (29.5 days).
The Far Side of the Moon Is Not Always Dark
People often call the side of the Moon we don’t see the far side of the Moon or the dark side of the Moon. While the far side is “dark” in the sense that we didn’t know what it looked like prior to photographs taken by the Soviet Luna 3 space craft in 1959, the far side experiences day and night. There actually are peaks of perpetual day near the northern pole and craters of eternal darkness at both lunar poles.
Why We See More Than Half of the Moon
While you might expect to only see half of the Moon from tidal locking, viewers from Earth see about 59% percent of the lunar surface. The effect is called libration. Libration results from a slight eccentricity of the Moon’s orbit around Earth, an inclination (6.7°) between the axis of the Moon’s rotation with respect to its orbit around Earth, and a small daily oscillation caused by the Earth’s rotation.
Earth as Seen From the Moon
If you stand on the Moon, the Earth stays in roughly the same place in the sky. The position of the Earth changes if you move elsewhere on the Moon. The Earth isn’t visible from the far side of the Moon, ever.
References
- Gladman, B.; et al. (1996). “Synchronous Locking of Tidally Evolving Satellites”. Icarus. 122 (1): 166–192. doi:10.1006/icar.1996.0117
- Lewis, John (2012). Physics and Chemistry of the Solar System. Academic Press. ISBN 978-0323145848.
- Mulholland, D.; Silverberg, E.C. (1972). “Measurement of Physical Librations Using Laser Retroreflectors”. Earth, Moon, and Planets. 4 (1–2): 155–159. doi:10.1007/BF00562923
- Touma, Jihad; Wisdom, Jack (1994). “Evolution of the Earth-Moon system”. The Astronomical Journal. 108 (5): 1943–1961. doi:10.1086/117209