# Why does the Earth Spin

Find out why our planet has rotational movement!

The story of why the Earth spins goes back to the formation of the solar system. In the beginning, roughly 4.7 billion years ago, the solar system was a large swirling cloud of dust and gas. Over time this gradually coalesced into stars and planets, being drawn into these shapes by gravity. This motion of being pulled inwards increased the angular momentum of the various bodies, and thus caused them to start rotating faster.

Consider an ice skater spinning with their arms outstretched. As they spin, they bring their arms inwards. Doing so increases their angular momentum, which makes them spin faster. The same is true for when the Earth first formed. As the dust and gas was compressed into one solid mass, the total mass of the object became more confined and subsequently began to rotate more and more rapidly.

The law of inertia states that anything stationary or moving with a constant speed wants to continue doing so until it is acted upon by another body/force. Considering the Earth rotates in space, which is a vacuum, there is nothing to drastically slow the Earth down which is why it continues to spin. Interestingly, early in its formation, the world spun up to five times faster than it does now – so we know Earth has lost some speed.

The culprit is the moon. It is our own natural satellite that has caused the planet to slow down, via something known as tidal locking. The moon at the moment is tidally locked to the Earth – that is, the same face always looks towards us, but it was not always so. When the moon first came into orbit around the Earth it was also spinning. To understand tidal locking, imagine that you and a friend both pull on a piece of rope, but at the same time you spin in a circle around a pivot at the centre of the rope. As you tug harder, you are eventually able to spin less and less fast. Eventually, you will be stuck simply pulling on the rope, unable to move sideways as your pulling force is too great; this is essentially what happened between Earth and the moon. As the moon orbits the world it exerts a pull on the planet, which is responsible for causing tides. , The Earth is much bigger so it continues to spin freely, but the moon’s rotation now matches the time it takes to complete one orbit Small as it may be, the moon will continue to have an effect on Earth and, millions of years from now, a day on Earth could be up to 26 hours long.

## Getting in a spin

The rate of rotation of a body is determined by the rapidity of its formation (ie a faster collapse means a greater angular momentum is conserved). Impacts from meteorites and the gravitational effect of natural satellites can eventually slow the body be it a planet or a star. In our solar system, the distance to the Sun also determines how fast a planet will spin – the closer a planet is, the slower it will go and vice versa.

This is an effect known as tidal locking, which is demonstrated by moons that are tidally locked to their host planets. They begin spinning but eventually slow down and finally are gravitationally locked, so the same face always looks towards their host planet, much like our moon.

The fastest spinning objects in the universe are pulsars. These are neutron stars that are left behind after a giant star goes supernova. Pulsars have a huge amount of mass confined into a very small space, sometimes less than a few dozen kilometres across. For that reason they have a very high angular momentum; some rotate up to 1,000 times a second.

## Facts about the Earth spinning

Inertia – The law of inertia states an object will continue to move unless acted upon by another force, which is why Earth has not stopped spinning.

Ecliptic – The Earth orbits the Sun on a flat plane but it does not rotate perpendicular to this plane.

Poles – The poles experience little to no rotational force, and thus can experience prolonged daylight and darkness in summer and winter, respectively.

Axis – The Earth rotates around an axis that is about 23.50 to the vertical line through the planet perpendicular to its orbital plane.

Angle – The angle of the Earth’s rotation has not always been the same; some research suggests it changes up to 1” every million or so years.

Seasons – The tilt of the Earth to the Sun determines the season experienced in each hemisphere, with a tilt towards or away leading to hot and cold seasons, respectively.