Re: Is Earth's Rotation Slow-Down a HOAX?

Apparently on date Sun, 08 May 2005 19:39:47 GMT, "larswilson"
<wilsonl035@xxxxxxxxxxxxx> said:

>>
>> AFAIK it is currently about 0.005 seconds a year, per year. The year is
>> the
>> same length of time, near enough, but there are slightly more days in a
>> year,
>> of the order of one second per 200 years, or that instead of 365.25 days a
>> year
>> we will have 366 days a year in about 13 million years.
>>
>Thanks so much for this information. Problem is, during the NB and Persian
>Period, the delta-T reduces at a rate of 3 minutes about every five years.
>Thus 12 hours from the Seleucid Period are basically reduced over 2500
>years. So is the tidal drag just during the Sleucid Period 3 minutes every
>five years? compared to the slow rate by the Egyptians in 1300 BCE and now?
>Furthermore, as noted, the Egyptians had the same number of days in their
>year quite similar to now. How is it that the Earth's speed sped up just
>for the NB Period?

Well I would guess that you have some incorrect interpretation of whatever data
you have indicating these variations in days per year.

Typo warning, by the way, when I say 366 above, I mean 364. The slower the
earth rotates, as time passes, the fewer days you can fit into a solar year.
When the earth was freshly formed, it span a lot faster. Maybe the best way to
look at this question is to consider the hours per day, and hours per year. Go
back to well before the dinosaurs, and you have a day that is 23 hours long,
rather than 24. There being 8,766 hours a year, you would then have 381 days
per year.

As the earth slows, a day will eventually be 25 hours long, and then we shall
only have time to fit 250 of them into a year, which will still be 8,766 hours
long unless something "interesting" happens.

And that's a key issue for you to consider. As you reverse time, tidal drag
makes the earth spin faster as you view the film rewinding, naturally, and as
you get back towards the early solar system*, you find the earth would become
so fast in its spin that stuff was almost being flung off into space, which is
the reverse film of stuff condensing into the earth (and other bodies) in the
accretion disc.

Now the thing is, if you have initial conditions four billion years ago like
that, and you have tidal drag from the sun and moon over a four billion year
period, you end up with an earth that has the same angular momentum as our
planet happens to have. It's a smooth curve related to rotation speed fitting
the present point, the apparent initial conditions near the birth of the solar
system.

This is all newtonian and therefore simple equations, e.g. Mercury experiences
a much larger tidal force and is much lighter, so long ago entered a form of
tidal lock with the sun. Jupiter is much larger and further from the sun so
retains a much larger percentage of its initial angular momentum. The tidal
forces themselves are quite easy to measure, it's the same as the gravity force
but weakened more rapidly by distance:

Sun mass 1.99E+30kg, distance 1.47E+11meters tidal force m/d^3 = 0.00063
Moon mass 8.83E+22 kg, 3.84E+8 meters, m/d^3 = 0.0015 (two / three times the
sun's effect.)

These together can shift three minutes of the year off over a period of 36,000
years.

The next biggest tidal force generator is Venus:
4.90E+24 kg / 3.74E+10 m = 9.37E-08 = 0.000000094
so you can ignore that from now on for this purpose.

To get a tidal force a hundred times as potent as the moon (plus sun), you need
something larger than earth, about as close as the moon is (and then it would
be basically earth sized and we would be orbiting around it as much as vice
versa.) Or something, say, a tenth of the distance the moon is, which throws
the moon out of orbit for a start, and it has to hang about for 2500 years
which sounds like a stable orbit to me so something later on has to kick it out
of orbit and leave earth / moon where they are.

Similar, I can't think of any way to speed the earth back up again. Sure, a big
enough impactor would add momentum if it hit right, but a little tiddler like
the K-T impactor had no effect whatsoever to either orbital period or rotation
period due to being much too small compared to the size of the earth.

I daresay there are theoretical ways of making what you describe happen from an
orbital maths point of view, but I think you'll find all the solutions will
leave the earth basically sterilised and having to start again with the
blue-green algae. And when you add into that the fact that the earth currently
has exactly the right day length to match what would be expected had none of
this happened for the last four billion years, then you have to think that
makes any of it unlikely. Whereas the idea that the sumerian / egyptian
accounts that it did, might be a misinterpretation or just false when written
down is an easy idea to entertain.

(* - complicated by an impact with a proto earth and object that together
formed the earth-moon binary, but this really just moves the initial starting
figures along a bit, as it was early enough and involved stuff reaching escape
velocity and forming the moon, which is basically the starting point anyway.
This aspect is compatible with, rather than an explanation for, the current
rotation rate.)







.

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