Our moon has a near side and a far side. One half of the moon always faces Earth, and one half always points away. Does this mean we can only see 50% of the moon’s surface from Earth? No. In fact, over time, it’s possible to see as much as 59% of the moon’s surface, due to a slight north-south rocking and east-west wobbling of the moon known as lunar liberation.
The time taken for the Moon to spin on its axis is almost exactly the same as the time it takes to orbit the Earth. Hence, the Moon always keeps the same side pointing our way. This is not a coincidence. Over billions of years, the Earth’s gravity has forced the Moon to spin synchronously with its orbit.
Here are the far and near side of the moon:
However, things are a bit more complicated than that. Viewed from Earth, the Moon appears to rock slowly backwards and forwards so that we see a slightly different face throughout the lunar month. There are two main reasons for this. First, the Moon’s orbit around Earth is elliptical not circular so its rotation is sometimes ahead, and sometimes behind, its orbital motion. And second, the Moon’s rotation axis is not at right angles to its orbit around the Earth so we can sometimes see ‘over’ or ‘under’ its poles. Over time this means we actually get to see about 59 per cent of the Moon’s surface.
This effect is known as tidal-lock. The “lock” 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 it’s orbiting.
The Moon’s 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 Moon’s 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 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, it’s still a measurable effect! The Moon is slowing down the rotation of the Earth by about 15 microseconds every year, gradually lengthening our days.
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