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Offline moccacake

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prehistoric gravity
« on: 23/04/2006 21:50:19 »
i just heard in some news that the earth rotated 16 hours a day 60 million years ago. so does that mean that if astronauts went back 60,000,000 years into the past the surface pressure will crush their bodies? can anyone pls. explain this to me? thanks.


 

Offline harryneild

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Re: prehistoric gravity
« Reply #1 on: 23/04/2006 22:03:55 »
Yes i heard this and also that t a year was 400 days. If i think about it would not crush us, but make us feel lighter due to an increase in centrifugal force competing with gravity.
Why do you think it would increase the surface pressure?

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« Last Edit: 05/04/2007 22:04:34 by harryneild »
 

Offline ukmicky

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Re: prehistoric gravity
« Reply #2 on: 23/04/2006 22:12:39 »
In the long distant past the earth did spin a lot faster than it does now, it has slowed down due to the moon robbing energy from the earth as it slowly moves away through tidal friction. as for the surface presure crushing our bodies due to the spin of the earth if we went back seems a bit over the top to me. i dont think the spin of the earth affects the air presure that much but i'm not an expert

Michael
 

Offline moccacake

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Re: prehistoric gravity
« Reply #3 on: 23/04/2006 22:18:00 »
quote:
Originally posted by ukmicky

In the long distant past the earth did spin a lot faster than it does now, it has slowed down due to the moon robbing energy from the earth as it slowly moves away through tidal friction. as for the surface presure crushing our bodies due to the spin of the earth if we went back seems a bit over the top to me. i dont think the spin of the earth affects the air presure that much but i'm not an expert

Michael



i just thought when you put a pilot in a centrifuge, the faster the centrifuge spins the pilot blacks out. i'm not sure about that.
 

Offline daveshorts

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Re: prehistoric gravity
« Reply #4 on: 23/04/2006 22:39:15 »
You are right Michael the earth is slowing down, the moon is robbing energy and angular momentum from the earth through the tides - if the moon pulls on the earth's oceans, the earth's oceans are pulling on the moon. So the earth's rotation is slowing down and the moon's orbit is getting larger.

They know that there were 400days in a year 200 odd million years ago, because geologists have found ancient corals that put down a small layer of shell every day, these were thicker in the summer than the winter, and if you look at the number of the small layers in a year you count about 400, which is neat.
 

another_someone

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Re: prehistoric gravity
« Reply #5 on: 24/04/2006 00:34:07 »
If you are saying that the last 200 million years lost us 35 days per year, then all things being equal, the next 60 million years should lose us only about 10 days per year, which (if I can do my maths right) should amount to a loss of about 40 minutes per day, not a loss of 8 hours per day.

Even if we do have a loss of 8 hours per day, the extra gravitational pull that would apply would be very slight – humans can survive 5 or 6g for a short period of time (even more in certain circumstances), and I would have though the loss of 8 hours a day rotation would not make any perceptible difference.

The bigger problem might be that if these astronauts had been wondering around in space for 60 million years, they may have become unused to any gravity whatsoever, and so even reacclimatising to 1g might be difficult.

As for going into the past, the effect of the Earth's rotation is to offset gravity (gravity pulls you down, the Earth's rotation will push to out), so the faster the Earth spins, the less you feel the effect of gravity.

As for how much of an influence this might be: there is no centrifugal force at the north and south pole, so the difference in effect the the centrifugal force has on our bodies today is the difference between the apparent gravitational force at the equator and that at the poles.



George
« Last Edit: 24/04/2006 00:38:28 by another_someone »
 

Offline Soul Surfer

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Re: prehistoric gravity
« Reply #6 on: 24/04/2006 10:36:28 »
I dont know why you are all talking about gravity which depends only of the masses of the objects involved and it is unlikely that any of the major objects have changed their mass significantly over thousands of millions of years.  all the changes are due to angular momentum exchanges between orbiting bodies.



Also your numbers don't work out  400 days in a year 200 million years ago sounds a possibility but this then depends on how much the length of the year may have changed and how much the length of the day.  The day is likely to be lengtheming due to tidal friction.  Any friction effects would conversely tend to shorten the length of the year although the efects of other planets can operate in either direction so I think that its a fair bet that over this relatively short period compared with the age of the solar system the length of the year has not changed much  now there are about 365 days in a year now and we have about 24 hours in a day so that is about 8768 hours in a year assuming that the year has not changed in its length if we had 400 days in a year the number of hours in a day would be  8768/400  =   21.9

Here are some other figures from

http://www.lpi.usra.edu/education/skytellers/day_night/about.shtml

Researchers examining ancient corals noted that annual growth patterns suggested there were more days in a year in Earth's distant past. Fossil corals, 380 million years old, from the Devonian Period recorded 400 daily cycles. About 290 million years ago in the Pennsylvanian Period, there appear to have been 390 daily cycles each year. Assuming that Earth's revolution around our Sun has not changed dramatically, this means that the number of hours per day has been increasing and that Earth's rotation has been slowing. Today's day length is 24 hours. During the Pennsylvanian Period a day was ~22.4 hours long. In the Devonian Period, a day was ~21.8 hours long. Earth's rotation appears to be slowing approximately 2 seconds every 100,000 years. Why are Earth's days getting longer? Some scientists suggest that tidal cycles create a “drag” on Earth, causing it to slow down.


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Offline daveshorts

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Re: prehistoric gravity
« Reply #7 on: 24/04/2006 11:00:52 »
Ok those look like the figures I was misquoting cheers
 

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Re: prehistoric gravity
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