The good news is that if the Earth’s rotation stopped, we wouldn’t fall off. With water pushed to the poles, we could walk on land around the entire equator, but it would be a very inhospitable place, as Dr Karl Kruszelnicki explains.
We know that the rotation of the Earth is gradually slowing down. But what would happen if God, the devil or aliens suddenly and completely stopped our planet from rotating on its axis of spin? Luckily, thanks to improved knowledge about our planet, geographers can now give us the answers.
Of course, if you suddenly stopped the Earth from spinning, most of our planet would rapidly become very inhospitable.
Half of the planet would almost continuously face the heat of the Sun, while half would face the cold of space.
Life could continue in a narrow twilight zone between the hot and cold halves. But this twilight zone would slowly creep around the planet over the period of a year, as the Earth did its annual orbit around the Sun.
To make it easier to work out what would happen, let’s pretend the oceans don’t freeze on the cold side, or evaporate on the hot side. And let’s look only at centrifugal force, which should really be called centripetal force.
Over several billion years, this force, which effectively pushes outwards, has made the planet a bit fatter around the middle. So the diameter of the Earth measured through the equator is today about 21.4 kilometres more than the diameter of the Earth measured through the poles.
But this bulge in the solid Earth took billions of years to slowly develop. This is because the solid matter moved only very slowly in response to the outward force caused by the spin of the planet.
But the liquid water in the oceans is far more mobile and responsive to forces. So the Earth’s spin has pushed up this liquid water to an ‘abnormal’ elevation of about eight kilometres.
In other words, at the equator, thanks to the spinning Earth, the water has been pushed up some eight kilometres higher than in the case of the Earth having no spin.
But today, on the entire equator, the deepest part of the oceans is only about 5.75 kilometres.
So take away the spin and you take away all water at the equator.
If the Earth were to stop spinning on its axis, gradually the oceans would migrate towards the poles from the equator. At first, only small regions of terra firma around the equator would rise out of the retreating waters.
Eventually, there would be a huge mega-continent wrapped continuously around the Earth at the equator. You could travel around the Earth on the equator and stay entirely on dry land—ignoring the freezing cold on the night side, and the searing heat on the day side.
The water that left the equatorial regions would have to go somewhere, and that ‘somewhere’ would be the poles. There would be two totally disconnected polar oceans on each side of the equatorial mega-continent.
In the north, Canada would be entirely underwater. And roughly following the line of the border of current-day USA and Canada, all of Greenland, as well as the northern plains of Siberia, Asia and Europe would be underwater. But Spain would mostly stay above water.
On the other side of the equator, the new southern ocean would start roughly on a line running through current-day Canberra. Africa would be joined to Madagascar, while Australia would be joined to New Guinea and Indonesia.
It turns out that the underwater basin around the South Pole is much bigger than the one around the North Pole.
So the new southern ocean would be lower. Because it’s a bigger ‘bowl’ with a greater capacity, its sea level would be about 1.4 kilometres lower than the sea level of the new northern ocean.
Now it’s not just the spin of the Earth that has given us today’s eight-kilometre-high bulge of water at the equator.
The other factor is gravity. The poles are about 10 kilometres closer to the centre of the Earth than the equator, so the gravity is ever so slightly stronger at the poles.
This would be another factor drawing the water away from the equator.
Our spinning Earth is in fact slowing down. Billions of years in the past, the faster-spinning Earth had a bigger bulge around the equator, and billions of years in the future, the slowed-down Earth will have a smaller bulge, and will be closer to a sphere.
In fact, this slowing of the spin is why we have to add an extra second into our clocks every 500 days or so. I’ll talk more about that, next time.