It was in the news this week that the earth's core, which does not rotate at the same speed as the earth as a whole, may be changing it's rotational speed, and that could possibly affect the length of a day. It might be a good time to review this posting.
The earth's molten iron core may seem remote to most people, and not an object of daily concern. But the iron core acts as a magnet, providing the earth with a magnetic field, which is why a compass works. The magnetic lines of force extended through space between the north and south magnetic poles. These lines of force are known as the Van Allen Belts, and protect the earth from the steady stream of charged particles from the sun. Without this the earth's atmosphere would be gradually stripped away.
What about the moon? Lacking such an iron core it has no such protection and has virtually no atmosphere. Although part of the reason for this is that the moon's surface gravity is less than earth's.
There have been news articles with opposite points of view on the changing length of a day. The majority of the articles were about the rotation of the earth speeding up, so that the length of a day increases by a few milliseconds.
The long term is, of course, that the rotation of the earth is gradually slowing down, as it has been for hundreds of millions of years. The friction caused by the moon's tidal effect on the earth's oceans is what has long been increasing the length of the day. The same process is causing the moon to gradually move further away from the earth. Some of the earth's energy of angular momentum is being transferred to move the moon to a higher altitude, which is a higher energy, orbit.
The simple example that is usually given is that of a spinning skater on ice. Angular momentum must be conserved and it requires more momentum for the skater to spin with arms extended because more mass must move through a wider circle. This means that the skater's spin will slow down when arms are extended and speed up when arms are drawn in.
The same concept applies to the earth's rotation. The more mass there is further from the center of the earth the slower the earth must rotate. If some of the mass is brought to lower altitude, closer to the center of the earth, then it's rotation must speed up. The moon is part of the earth's gravitational system and, by the same principle, having the moon move further away means that the earth's rotation must slow down.
An obvious way to explain why the earth's rotation is speeding up, if only by a tiny fraction of a second per day, is the melting of glaciers caused by global warming. The water flows into the sea, which is a lower altitude, and this causes the rotation to speed up.
One thing that I haven't seen referred to is how the burning of fossil fuels, aside from the global warming that it causes, might contribute to speeding up the earth's rotation. When oil or coal is under the ground it is more a part of the earth's mass than when it is loose atoms in the atmosphere, after burning. This could cause the earth's rotation to speed up. In addition, rampant deforestation across the world means that more carbon atoms will remain loose in the atmosphere, rather than being incorporated into the structures of plants.
While overall the rotation of the earth should continue to slow down the idea is emerging that humans are capable of affecting it, one way or the other, if only by a few milliseconds per day.
Here is a link to "Slowing The Rotation Of The Earth":
www.markmeekprogress.blogspot.com/2009/06/slowing-rotation-of-earth.html?m=0
One other thing that I have not seen much written about is what it was like a very long time ago on earth when the earth spun much faster and the moon was much closer, both of which would have magnified the effects of the tides? It is generally agreed that life began in the sea, before moving onto land. Could this have been brought about by much greater tides than we have today?
Here is a link to the posting, "Life Without The Moon":
www.markmeeklife.blogspot.com/2009/07/life-without-moon.html?m=0
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