We all learnt at school, that the Earth rotates on its axis, giving us night and day. We may think the length of a day has not changed, but it is getting longer by a scientifically measurable amount. However, the ultimate reason for the Earth to stop spinning will be its inevitable destruction when our sun turns into a red giant.
To understand what is causing the lengthening of the day, you need to look at the Earth as a whole. If you saw it from space, you would just see a spherical globe gently turning. As it’s in space, there will be no frictional forces acting on it. We have all spun a top, and watched it spin on its point. But we know that eventually it will slow down and fall over. This slowing is due to friction against the table and the air. There is no such friction acting on the Earth as it spins gently in space. However, there is another body close by – the moon.
The moon is about 250,000 miles from the Earth, and is locked in an orbit of 29.5 days. The moon is massive enough to have an effect on the Earth’s surface, most noticeably resulting in the tides of the sea. One of the high water marks is almost directly under the position of the moon. So, as the Earth turns relatively quickly, the position of the tides changes. Now this change is resisted by the frictional forces of the seas moving against shorelines and over the sea floor. These frictional forces have 2 effects. The first is that the Earth rotation slows down, and the second is that the moon is moved away into a higher orbit. This second point may not be obvious, but it is caused by the conservation of angular momentum – the same reason an ice scatter will spin at different rates if she holds her arms in tight, and then reaches out wide.
Now, as the moon gets further away (about 2 cm per year) its effect on the Earth’s oceans decreases. Therefore the moon by itself cannot stop the rotation of the Earth.
Incidentally, the same rotational slowing has been acting on the moon, caused by the Earth itself. This time, the Earth’s pull is enough to distort the actual shape of the moon, resulting in a tidal movement in its crust. This has been enough to stop the moon’s rotation – the same side of the moon always faces the Earth. This is called tidal locking.
So, although the rotation of the Earth will slow, it will not stop – until that is, our sun becomes a red giant. In about 4 billion years, our sun will run out of its hydrogen nuclear fuel. When this happens, the outer gas layers will expand and cool, turning from ‘yellow’ hot, to just ‘red’ hot. The sun will still shine, as new nuclear reactions will start in the core. However, the outer expansion will be so great as to swallow all the inner planets, including the Earth. When this happens, the Earth will not only stop rotating, but will also be destroyed.