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Author Topic: Is the Earth spinning more slowly? Were days shorter in the past?  (Read 6164 times)

Offline L33zy

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Ok, my information on this is a little spotey so bare with me...  I heard that the earth slows down in its rotation around its axis each year, and that this is the reason we must be constantly changing our clocks for accuracy.  My question is, that if the world is slowing down in its rotation, and has been since the big bang, A.) approximately how fast was it going originally

B.) How did this effect the early development of life on the planet.

Just wondering because it seems like if it has been slowing down each year, even if it is just a little, over a billion years that would make it rotate substantially faster.
Any information on the subject would be most appreciated.
« Last Edit: 27/06/2007 20:19:53 by chris »



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The Earth did not exist at the time of the big bang, so the timescales there are a bit wrong.

The main reason the Earth's rotation slowing down is that energy is being transferred to the moon (i.e. it is not clear how much it would have slowed down without the moon being there).

Certainly, the day's length at the time of the first time life arose on Earth was much shorter (and the moon was closer to the Earth, thus the tides would have been stronger, and the month also would have been shorter when measured in terms of the annual orbit of the Earth around the Sun).
The Earth's day has increased in length over time. The original length of one day, when the Earth was new about 4.5 billion years ago, was about six hours as determined by computer simulation. It was 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone). This phenomenon is due to tides raised by the Moon which slow Earth's rotation.

No doubt this would have had a significant effect upon life on the early Earth, but I doubt anybody could presently say what that effect was.

Offline L33zy

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no, actually I think the slowing is based on the space dust that the world is constantly going through, although the moon may play a factor in that also.  As for the big bang, I was making a hyperbole ;)  I just meant as the world was in development in location in the solar system.

Offline daveshorts

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I think george is right the slowing of the spin due to the tides acting from the moon and the sun will be much larger than space dust. The Space dust may be a larger contributor to any energy lost from the earth's orbit


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The Space dust may be a larger contributor to any energy lost from the earth's orbit

Even with regard to the Earth's orbit around the Sun, would there not be substantial tidal forces between the Earth and Sun (even if less than that between Earth and its moon, but enough to be more significant than space dust).

Furthermore, the space atmosphere the Earth is travelling in is a supersonic solar wind, and as such, if it has any effect on the Earth's orbit, it should do as much to push the Earth away from the Sun as it would to decrease its orbital speed (although inevitably, the further the Earth is from the Sun, the slower its orbit is going to have to be if it is to maintain a stable orbit).

Offline DoctorBeaver

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L33zy was asking about the earth's rotation about its axis, not its orbit around the sun.


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I'm not sure that space dust would have any slowing effect. It's hitting the whole face of the Earth, so half would be speeding it up and half would be slowing it down(?).
The tidal forces between Sun and Earth are very  significant - they account for the difference between spring and neap tides.

Offline chris

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Absolutely spot on.

In terms of why the Earth's rotation should be slowing down, the effect is effectively frictional. The Moon's orbit around the Earth moves massive amounts of water around the planet. Frictional drag on the water (as it rubs against the ocean floor, the coastline or itself) causes the movement of the water (known as the tidal bulge) to lag slightly behind where the moon is trying to move it.

This misalignment causes a torque between the Earth and the Moon which acts to decelerate the Earth's rotation and increase the Moon's angular momentum. This results in a slight increase in day length estimated to be about 2 milliseconds (two thousandths of a second) per day every 100 years.

This means that 100 million years ago (when the dinosaurs were trotting about) a day would have been roughly half an hour shorter than today - so a day would have lasted 23.5 hours (0.002 sec/day/century x 1,000,000 centuries = 2,000,000 msec (2000 seconds) = 0.55 hours. A year at this time would have lasted 374 days.

Looking further back in time, 600 million years ago the day would have been 3.3 hours shorter, giving 424 days/year.

The point was also raised regarding the effect this might have had on animal and plant physiology. The answer is that animals and plants would almost certainly have had different settings on their body clocks. We know that pretty much every species on earth (including even micro-organisms) show signs of having a body clock which runs on a kind of genetic domino effect where one gene switches on and activates a second, which activates a third, which feeds back and switches back on the first and so on. This process is set up to take 24 hours. Plants have a form of it too. If we changed the length of the day then these gene cycles would evolve over time to accommodate this change in day length. Indeed this is exactly what we see.

The exception to this rule would be if we were to take plants from Earth to a different planet. I asked Alex Webb about this when he appeared on the Naked Scientists programme:

If we took plants to Mars, for instance, the different day length would cause them to grow sub-optimally. A key to successful space colonisation is therefore to understand these plant growth patterns so that we can work out how to alter them to make plants compatible with our future new cosmic habitats.

« Last Edit: 01/07/2007 15:29:00 by chris »

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