why do we see only one side of the moon

Why does the Earth only see one side of the Moon? (I know about sychronous orbit, I'm just asking why)
The Moon rotates exactly once per revolution of the Earth which is the reason why it shows only one face at the Earth. If you face a pole and move around the pole showing your face at the pole all the time, you will notice that you have rotated once per revolution around the pole. Why has this happened to the Moon? It is due to tidal forces of the Earth. You know that the Moon's tidal forces causes the high and low tides on the Earth. In the same way, the Earth exerts tidal forces on the Moon which are more powerful as the Earth is more massive than the Moon.


It turns out that these forces exert torques on the rotating system and tends to slow its rotation till it finally shows the same face towards the other body. Hence, it is the effect of tidal forces of the Earth on the Moon that have caused the Moon to show only one face to the Earth. Similarly, the tidal forces of the Sun on Mercury have slowed down its rotational period to once in 59 days. The Moon's tidal forces will have the same effect on Earth, so that some day in future (billions of years hence), the Earth may show the same face to the Moon. This page was last updated on July 18, 2015. 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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