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What makes the Earth spin on its axis every day? – Reid, age 5, Melbourne.
To answer this tricky question, we have to look back in time to when the Earth was born, 4.5 billion years ago. We have to ask: why did the Earth start spinning in the first place?
When our solar system formed out of a gas cloud, called a nebula, there was lots of dust and gas coming together due to the force of gravity. The dust and gas was already moving around in a circle. As it all clumped together to form the Sun and planets, these new objects started to spin – and then spin faster.
Read more: Curious Kids: Why do our ears pop?
How the spin sped up
When you make a spinning object more compact, it spins faster. It’s just like how an ice skater or a ballerina brings in his or her arms when they want to spin faster.
So when all of the rocks in the clump of dust and gas started coming together, that made the Earth’s spin speed up.
We’ve covered what made the Earth start spinning, and what made it pick up speed. But why does it keep spinning?
Why the spin continues
In theory, a spinning object will just keep spinning forever unless you add energy or take it away.
Imagine a spinning top. You added energy to it by starting the spin off with your hand. Eventually it stops because the ground it is spinning on is taking energy away from the toy top through something called friction.
Friction is where something rubs on or drags on an object and takes energy away. Have you ever slowed yourself down while you’re riding downhill by dragging your foot on the ground? That’s friction.
There’s not much friction in a fidget spinner toy. That’s why they can spin for so long.
But eventually, even the air around you can cause friction and slow down a spinning top or a fidget spinner.
Back to Earth
Now imagine the Earth floating in space. It will keep spinning unless something slows it down. It would take a LOT of energy to slow down the spinning Earth because it is so big.
It’s not spinning on the ground, so the ground won’t slow it down. There’s no air outside our own atmosphere to slow it down either. That is why the Earth has continued to spin for a very long time.
However, some things are slowing the Earth down or could change its spinning in the future.
You might have heard that the Moon causes the waves and tides in our oceans. Because of this, the Earth is slowing down very slowly (about 1 second every 50,000 years). As a result, the Moon is also slowly moving away from us.
When the Earth and Moon were very new, the Moon was much closer (in fact it is thought that the Moon was once part of Earth but they came apart during an explosive collision with a huge asteroid.)
How do we know that? Well, scientists examining rocks realised that there used to be many more days in a year. In the age of the dinosaurs, a day was 22 hours rather than 24 hours that it is today.
The Sun’s gravitation also causes the Earth to slow down a bit too. And if an asteroid or comet hit the Earth, that might speed us up, slow us down or even knock us over (scientists think this may be what happened to Uranus, which is actually on its side - you can tell by the vertical stripes the clouds make rather than the horizontal ones you see on Earth, Jupiter and Saturn).
Here are some more cool planet spinning facts:
All the planets have different day lengths. Mercury’s day is about 59 Earth days, while Jupiter’s day is only 10 Earth hours long.
Venus has the longest day (if you define day as meaning one full spin) of nearly 117 Earth days. It actually spins in the opposite direction to all of the other planets except Uranus.
These different day lengths are because of how the planets were formed and what knocked into them when they were very young.
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Jacinta den Besten does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
Authors: Jacinta den Besten, First Year Physics Coordinator, University of Melbourne