What keeps the earth spinning in space?

A friend recently recently asked me this intriguing question - what keeps the earth spinning in space? Though it sounds like a simple question, the answer depends on our understanding of a complex process like the formation of the solar system. But the simple answer to this question is angular momentum and inertia. Let's see how the earth's rotation depends on these two terms from classical physics.

Why does the earth spin? - Law of conservation of angular momentum

Let's briefly look at the early days of our solar system. About 5 billion years ago, a giant star, several times larger than our very own Sun went supernova. Most of the gas from this massive explosion clumped together to form the sun. The remaining gas and dust which are made of hydrogen, helium and small quantities of other heavier elements spread across the present day solar system.

Initially the gases were very hot and moved away from each other (hot objects have greater kinetic energy, which means, greater velocity). With time, they cooled down and started to coalesce, due to gravitational attraction, to form clumps that were irregular in size. These clumps, due to mutual gravitational attraction, started rotating around the center of mass. This is where angular momentum comes into the picture. When two objects, that are moving relative to one another, interact by exerting a force on each other (here, it is a gravitational force), they gain angular momentum and start rotating around the center of mass. The direction of rotation depended upon the net angular momentum of the all the moving clumps. The irregularity in the size of the clumps resulted in these clumps coming closer together to form even bigger clumps and eventually give rise to planets.

Credit: NASA/JPL

Due to the law of conservation of angular momentum, the eventual planet retained the angular momentum of the rotating clumps. This is why the earth rotates on its axis.

Animated gif by Wikiscient available under a Creative Commons Attribution-Noncommercial license

When I said that moving bodies, influenced by the force of gravity, would end up rotating around the center of mass, the question that might have popped up in your head would be "why does the earth go around the sun? Shouldn't the earth and the sun together go around a common center of mass?" In fact when we say that the earth goes around the sun, we are only partially correct. In reality, both the earth and the sun go around the common center of mass. If you do the math you will find that the center of mass is located at a distance of 500 kilometers from the center of the sun, which is well within the sun as the sun's radius is 700,000 kilometers. Therefore the sun does go around this center of mass but we don't observe it explicitly because it is within the sun and also because the other planets in the solar system also attract the sun.

So now we know why the earth started spinning. Let's go back to the original question.

Now what keeps it spinning? - Inertia

Newton's first law of motion says that an object remains in a state of rest or uniform motion, unless acted upon by an external force. This means that the earth was set spinning when it formed and it will remain to do so forever. This is what we call inertia. Inertia will keep the earth spinning forever.

But this doesn't sound right. On Earth, no spinning object keeps spinning forever. Why so? To understand this, we must isolate the most important words in Newton's first law, which are "external force". Let me explain why so. The two reasons why a spinning top comes to rest on Earth are friction and air resistance (negligible but it can play a small part). These two are the external forces. But it is well known that in space there is neither air resistance nor friction. Therefore it seems reasonable to believe that the earth will keep spinning until you consider the effects of gravity. The same force of gravity that imparted angular momentum to the earth can completely stop the earth from spinning. This gravitational effect is called the tidal force. This is the same force that causes tides in the ocean. For more details about tidal forces watch this video.

As mentioned in the video, tidal forces can cause dissipation of rotational energy in a body which has already happened in the moon due to earth's gravitational force. This is why the moon shows the same face to the earth all the time. Sometime in the far future, the moon will return favors and the earth will also stop rotating, becoming tidally locked to the moon. This means that you would be able to see the moon only from one side of the earth. That would be sad. So make sure that you catch a glimpse of the moon and enjoy its beauty while it's possible.

To summarize the points made in this post

- Why did the earth start spinning? - due to the angular momentum of the gas and dust clumps that formed the earth.
- Why is the earth still rotating - Inertia
- Will the earth spin forever? - No, the tidal forces on Earth's surface due to the moon's gravity will slowly deplete the earth of all its rotational energy and it will stop spinning.