Download Rotation of the Earth

Gravity
The question
If the world were to stop rotating, would there still be gravity? Would we still be able to live
on our planet at all?
J. Zwols, Groningen
The answer
This is in fact two questions.
1. What is the connection between gravity and the rotation of the earth
The earth rotates in a few different ways. It turns around its own axis (in approximately 24
hours), it revolves around the sun (in approximately 365 days), the solar system rotates
within the Milky Way and the Milky Way too makes a turning motion around a centre.
Two different kinds of motion are to be discerned (not just for the earth but for all celestial
bodies, albeit in different lengths of time): the axial rotation and the rotation in an orbit
around another celestial body.
The orbit around another celestial body (like the earth around the sun, but also the moon
around the earth etc.) is caused by gravity. Therefore, gravity is not the outcome but the
cause of this orbit. Gravity is the attractive force between two masses and dependent on the
distance in between.
The earth revolves around the sun at great speed. This is a lateral (centrifugal) movement.
The direction of that movement is at right angles to the falling motion towards the sun.
Without this gravitational force of the mass of the sun the earth would shoot into outer
space. Without the lateral movement the earth would crash onto the sun. The balance
between gravity and the lateral movement keeps the earth in its orbit.
Where then does this motion come from? In order to answer this we need to go back to the
formation of the solar system, about 4.5 billion years ago.
Our solar system was created from a giant interstellar cloud of gas and dust that in some
places started to cluster because of gravity. Through the clustering of a piece of that cloud
into our solar system the rotation was locally reinforced. A disk was created in which the
planets gathered together with the sun in their middle. On top of that the axial rotation was
retained in the rotation of the sun and planets, including that of the earth. Something that
turns (without brakes) will keep turning. This means that the earth revolves around its own
axis owing to the conservation of the angular momentum.
2. What happens if the earth stops rotating around its own axis
Luckily for us this will not happen anytime soon. Should the earth all of a sudden stop
rotating, all loose matter would fly off, even the oceans and the atmosphere, because
everything is subject to the angular momentum.
Should the earth stop rotating slowly, day and night would increasingly last longer leading
to enormous influences on the climate and quality of life. Gravity will not disappear but, on
the contrary, will gain the upper hand because the compensating centrifugal power
disappears. This means that the centrifugal power will be strongest on the equator (after
all, rotation is stronger there than at higher degrees of latitude). On account of the
disappearance of the centrifugal power ocean water, which is now pulled towards the
equator, would move towards the poles, which would make the world look extremely
different.
However, there is no reason to be afraid of such a scenario any time soon. The earth is
decelerating very slowly under the influence of the moon, but a significant change in the
day’s length will not present itself for billions of years.
An interesting book, in Dutch, about this (suitable for all ages): Jos van den Broek (2013)
Wie is de baas van de zon, published by Micromys (only available in Dutch)
Kind regards,
C.M. Ree, MSc.
Science Shop
University of Groningen
Translated by Anniversary Project Group