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why do we only see one side of the moon

If you watch the way the moon spins (or doesnБt), you can see that only the left side has a consistent side facing the surface of our planet (which, we must note, is not even a little bit to scale here). However, the underlying reason why the moon rotates at this exact speed, forcing us to only see a single side of it, is because the moon has been tidally locked to the earth. Tidal locking is a stable configuration, and relatively easy to get to, given enough time, so many of our solar systemБs moons are found to be tidally locked, including the dwarf planet Pluto and its largest moon Charon, which are both tidally locked to each other. The БlockБ part of this name refers to the way that an object - like the Moon - is apparently fixed in position, with one side always facing the other object. Any object which is found to be tidally locked will always have one side of itself facing the surface of the planet itБs orbiting. The amount of time it takes to orbit around the planet will vary from object to object (Phobos, one of the moons of Mars, is tidally locked and orbits Mars every 8 hours - way faster than our Moon), but as long as the object is tidally locked, the rotation will match the length of time it takes to orbit.

However, itБs the БtidalБ part of the tidal locking that gives us the real key to why tidal locking happens at all. WeБre most familiar with tides as the effect of our oceans rising and falling due to the position of the moon. The MoonБs gravity pulls on the earth, and the water on the surface of the Earth closest to the moon responds to that pull by elongating towards the moon. The water on other parts of the earth feels the MoonБs gravitational pull as weaker, with the water on the opposite side of the earth feeling the weakest pull. However, these tidal forces also have another effect - they resist rotation. The Moon was almost certainly not tidally locked when it first formed - at that time, it would have rotated at a faster speed, which meant that had any observer been on the early Earth, they could have seen all sides of the moon as it spun.

However, the gravitational pull from the Earth - which like the tides due to the Moon, pulls on the side of the Moon closest to the earth more than the far side, resisted this faster rotation. This resistance due to the gravitational pull of the Earth gradually slowed down the faster spin of the Moon until the Moon was no longer rotating faster than it was orbiting. Once the MoonБs rotation had slowed so much that a single face was always facing the surface of the Earth, it had officially been tidally locked, and has stayed in this configuration ever since. The Moon also has the same influence on the Earth, but since the Moon is so much less massive than the Earth, this resistance to rotation takes a much longer time to impact the Earth's spin. However, itБs still a measurable effect! The Moon is slowing down the rotation of the Earth by about 15 microseconds every year, gradually lengthening our days.
The image of the Moon here is drawn as is normally shown on maps, that is with north on top and west to the left.

Astronomers usually turn the map over to have south on top, as to correspond with the view in most telescopes which also show the image upside down. West and east on the Moon are where you would expect them, when standing on the Moon. But when we, on Earth, see the Moon in the sky, then the eastwest direction is just reversed. When specifying coordinates on the Moon it should therefore always be mentioned whether (or rather ) coordinates are used or coordinates. The actual orientation you see the Moon in the sky or on the horizon depends on your geographic on Earth. In the following description a few typical cases will be considered. On the north pole, if the Moon is visible, it stands low above the horizon with its north pole up. In mid northern latitudes (North America, Europe, Asia) the Moon rises in the east with its northeastern limb up (Mare Crisium), it reaches its highest point in the south with its north on top, and sets in the west with its northwestern limb (Mare Imbrium) on top.

On the equator, when the Moon rises in the east, its N S axis appears horizontal and Mare Foecunditatis is on top. When it sets in the west, about 12. 5 hours later, the axis is still horizontal, and Oceanus Procellarum is the last area to dip below the horizon. In between these events, the Moon reached its highest point in the and then its selenographic directions are lined up with those on Earth. In mid southern latitudes (South America, South Pacific, Australia, South Africa) the Moon rises in the east with its southeastern limb up (Mare Nectaris), it reaches its highest point in the north with its south on top, and sets in the west with its southwestern limb (Mare Humorum) on top. On the south pole the Moon behaves as on the north pole, but there it appears with its south pole up.

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