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Astronomy Dark side of the moon
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Astronomy
Dark side of the moon
Why do we always see the same face of the moon?
The Moon spins like the rest of the planets in our solar system, but it spins at exactly the same rate as it moves around the Earth. In other words, each time the Moon goes around the Earth, it spins one time, with a period of 29.5 days. If it did not spin, we would see the whole of the moon over each month. Because of the way it spins, we only ever see one side.
This is not a coincidence and it has not always been the case since the Earth and Moon were formed. Just as the Moon creates a tidal pull on Earth, resulting in the movement of water in the oceans, the Earth creates an even bigger tidal pull on the moon. These tides result in a loss of energy, partly from the Moon's spin, which over time have slowed it down to the situation today, when the Moon's rotation has become tidally locked to its orbital period.
This phenomena is also called synchronous rotation, and it is found in most, but not all, of the planetary satellites(moons) in our solar system such as Deimos (Mars), Ganymede (Jupiter), Titan (Saturn), Ariel (Uranus) and Triton (Neptune). The planet Pluto and its moon Charon are the only example of mutual tidal locking in the solar system – they both spin at the same speed and always show the same face to each other.
If our moon's rotation slowed even more, say to 29 or 30 days, then the tides would transfer energy from the orbit to the spin and it would accelerate back to 28 days (and it would get closer to the Earth).
There are many websites that explain tides and the rotation of the Moon, such as:
The Astronomy Corner
Students for the Exploration and Development of Space (SEDS) founded in 1980 at MIT and Princeton Universities, USA.
The Internet FAQ Archives
Actually, the Moon wobbles slightly due to its slightly non-circular orbit, so that a few degrees of the far side can be seen from time to time, however the majority of the far side was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959.
Some people call the far side “The Dark Side of the Moon”. It actually receives as much light as the rest of the moon, but we cannot see it from Earth.
“The Dark Side of the Moon” is also the name of an excellent music album by Pink Floyd - one of the world’s greatest ever rock bands.
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