Question
Why do people on Earth only see one side of the moon?
Answers
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Answered by
GPT-5 mini
Because the Moon is tidally locked to Earth — it rotates on its axis once in the same time it takes to orbit Earth (about 27.3 days). That means the same hemisphere of the Moon always faces us.
How that happened (briefly):
- Early on, Earth’s gravity raised tidal bulges on the spinning Moon.
- If the Moon rotated faster than its orbit, those bulges were pulled slightly out of alignment, producing a torque that slowed the Moon’s spin.
- Over time, energy was dissipated and the spin slowed until the Moon’s rotation period matched its orbital period. That stable 1:1 resonance is called synchronous rotation or tidal locking.
Why we can sometimes see a little more than exactly one face:
- The Moon’s orbit is slightly elliptical and its axis is tilted a bit, so it wobbles (libration). That lets Earth observers see about 59% of the lunar surface over time, not just 50%.
“Dark side” myth:
- The “far side” (the hemisphere we don’t usually see) is not permanently dark — it gets day and night just like the near side. “Dark side of the Moon” is a misleading name.
Tidal locking is common for moons; Pluto and its moon Charon are actually mutually locked so they always show the same face to each other.
How that happened (briefly):
- Early on, Earth’s gravity raised tidal bulges on the spinning Moon.
- If the Moon rotated faster than its orbit, those bulges were pulled slightly out of alignment, producing a torque that slowed the Moon’s spin.
- Over time, energy was dissipated and the spin slowed until the Moon’s rotation period matched its orbital period. That stable 1:1 resonance is called synchronous rotation or tidal locking.
Why we can sometimes see a little more than exactly one face:
- The Moon’s orbit is slightly elliptical and its axis is tilted a bit, so it wobbles (libration). That lets Earth observers see about 59% of the lunar surface over time, not just 50%.
“Dark side” myth:
- The “far side” (the hemisphere we don’t usually see) is not permanently dark — it gets day and night just like the near side. “Dark side of the Moon” is a misleading name.
Tidal locking is common for moons; Pluto and its moon Charon are actually mutually locked so they always show the same face to each other.
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