What makes the world go round?

Ever heard the old corny saying, love makes the world go round? Well I was thinking about it today (thanks to today’s word prompt, lovingly) and realized that I actually didn’t know what really made the world go round. I had to look it up. The answer was surprising.

Nothing. The world just keeps spinning round because nothing has stopped it.

This strange statement requires some explanation. Our Earth spins about its axis every 24 hours leading to the passage of nights and days. No force acts on it to make it spin like this. It just always has as a result of the way it was formed.

To understand this, there is something called the law of conservation of angular momentum in physics. You may vaguely remember from your school days, something about Newton claiming that a body at rest or in uniform motion tends to remain in so unless an outside force compels it to do otherwise. Well, the same is true for spinning bodies. A body that starts spinning tends to go on spinning the same way unless some force causes it to change.

The Earth started spinning as a result of the how it was formed. Once it started, there was no force that was trying to stop it, so it has continued to spin to this day. So what caused it to start spinning in the first place?

Scientists believe that the solar system was formed when a dense cloud of gas started coalescing under its own gravity. As the gas coalesced, it started spinning till it formed a kind of bulbous disk. The stuff at the center of the disk formed the sun, while the rest of the gas coalesced into planets. Since all the gas was in the form of a disc, all the planets formed from the gas also orbit in the same plane.


Artist’s rendering of the early Solar nebula

But also, as the gas coalesced and became denser, it also started spinning to conserve its angular momentum. If this is not clear, bear with me, as the key idea is that when mass coalesces closer to the axis of rotation, its spin increases.

To understand this practically, sit on an office swivel chair (make sure it is well-greased and spins fast). Spin it around as fast as you can. Then when you are spinning at a fair rate, spread out your arms and legs. This spreads your mass away from the axis of rotation (the swivel axis of the chair) and your spin will slow down. On the other hand if you tuck in your arms and legs, you are bringing your mass closer to the axis of rotation and your spin speeds up.


Spinning chair experiment

The same effect is seen when an ice skater tucks in his arms, he spin becomes faster. Here is a short online video I came across showing this.


The basic idea is that when the mass comes closer to the axis, the speed of rotation increases. So when the gas formed a spread out disk, its mass was spread over a wide space away from the axis of rotation. There was a slight spin because gas at different distances from the sun rotated at different speeds.

As the gas coalesced and formed a ball, the mass of the spread out gas became concentrated and hence the spin also increased to the speed of rotation we see today.

Viewed from above the North Pole, all the planets revolve counter clockwise around the sun. Except for Venus and Uranus, they also rotate in the same direction, that is from west to east. This is speculated to be the direction that the original gas cloud rotated. Venus goes around the Sun every 225 Earth days but actually spins clockwise completing 1 rotation in 243 days. Uranus’s axis of rotation is tilted 90 degrees from its orbital axis, so it isn’t clear if it had a clockwise or counter clockwise spin.

In both these cases, it is believed the cause is later collisions the planet suffered after being formed. The Earth is also believed to have suffered a great collision, resulting in the formation of the moon.


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