The Earth's rotation has been progressively slowing down for billions of years. It's a process that's still going on now, with estimates indicating that the duration of a day is increasing by around 1.8 milliseconds per century. The research has revealed that the rotation of the Earth is in continual, microscopic flux. Where there is fluid on the surface of the planet, such as water or gas, it can move relative to the solid part of the world and cause the Earth to rotate differently at different locations. This moving fluid is called a "flux core".
In order to measure this slowness to a high degree of accuracy, scientists use satellites orbiting the Earth. These satellites contain sensors that monitor radio signals sent out by atomic clocks on the ground. The clocks in the satellites keep time using hydrogen atoms found in water molecules. As the Earth rotates, these water molecules flow toward the center of the planet where they get pulled into its interior. The clocks stop running during these times when the satellite is in daylight savings time and start again when it enters standard time. By comparing the times when different satellites reach their destinations, we can work out how long it takes for them to travel around the Earth.
This study was published in 2004 in Nature magazine. Since then, further research has confirmed and explained some of the findings made in this first study. For example, we know now that the slowdown is not uniform across the globe.
Every 25–30 years, the Earth's rotation slows down by a few milliseconds every day for around 5 years. The Earth's rotation slowed for the fourth year in a row in 2017. It is expected to slow down further in 2018 and 2019 before speeding up again.
The Earth's rotation is important because it gives us rotational momentum, which is used by engines and motors. Without this momentum, these devices would not be able to rotate like a wheel does. So, how does the Earth's rotation affect them? Engines and motors that use rotational energy need something to turn against, otherwise they would just continue turning forever. The Earth provides this thing called "angular momentum" which is the measure of its rotation. As angular momentum increases, so too does the efficiency of devices using this type of power source.
People have wondered about the end of the world since the beginning of time. Will the Earth stop spinning one day? Would we as humans cease to exist if the Earth stopped spinning?
Actually, no! The Earth will always spin until either the planet itself collapses under its own weight or it is destroyed by another major event such as being hit by a large asteroid or collapsing under the stress of overloading its axis of rotation.
A day was only around 22 hours long hundreds of millions of years ago, due to a planet rotating faster around its axis than it does now. It is expected that by the end of this century it will be getting more slowly again.
Spinning objects like wheels or drums will eventually stop turning if nothing else touches them. This is called "spin drift." As the Earth spins, its surface passes under different parts of the atmosphere at varying speeds. The amount of wind friction against the ground changes which influences how fast or slow the Earth rotates. Over time, wind friction slows down the Earth's rotation enough for the magnetic field it creates to fade away.
The last major ice age ended about 10,000 years ago. At that time, most of North America was covered in ice sheets hundreds of miles wide. The only part of the continent not frozen over was the middle tropics where there was grass and some trees but no water because it was too far from the oceans.
During an ice age, large areas of land are covered in ice. As the world starts to warm up after an ice age, these ice sheets start to melt which causes rivers to flow again and leaves our planet warmer yet again.
The Earth normally revolves on time around its axis, with one rotation lasting exactly 86,400 seconds (1,440 minutes or 24 hours). Our planet spins at a slower rate. The globe has ceased slowing down and is currently speeding up as of 2020. A day is now half a millisecond shorter than 24 hours.
This change in rotational speed is called "precession". The current rate of precession is about 1 degree per century, which means that if you started counting at any time in the past 10,000 years, you would end up with a different date each time because it's changing all the time.
The reason for this slowdown is that the axis of rotation is not fixed in space, but instead wanders within the Earth's crust. This movement produces the pole shifts observed from time to time. There are also other factors involved, such as the mass of the moon causing changes to the Earth's moment of inertia, but these are beyond the scope of this question.
Precession was important in determining when people began to live in cities because it affects when crops grow seasonally. If you wanted to plant corn every year at the same time, you needed a marker to indicate when it was supposed to be planted. Before clocks, this was done by looking at the Moon. If it was full, then it was time to plant; if it was new, then wait until next month.