Re: Earth rotation

From: Robert Grumbine (bobg_at_radix.net)
Date: 06/18/04 

Date: Fri, 18 Jun 2004 20:21:02 -0000

In article <273f8e06.0406180842.45ba6f99@posting.google.com>,
Oriel36 <geraldkelleher@hotmail.com> wrote:
>don@tower.net.au (don findlay) wrote in message
>news:<5f164087.0406150729.593b9353@posting.google.com>...
>> "N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox T:net@nospam.com> wrote
>in message news:<Pkizc.3$ey.0@fed1read06>...

[large snip]

>> > Given that the Earth is a thin veneer of crust over a liquid center, the
>> > 'pivot' is located at the rotational center of gravity. For some period of
>> > time it has passed through/near the current geometric North and South
>> > poles.
>>
>> Trust you mean by 'pivot' the *barycentre* of the Earth and Moon? And
>> what might that have to do with the thin veneer of skin getting out of
>> kilter with the 'liquid'? (I take it you mean that the crust and the
>> 'liquid' would decouple/ detach?)
>>
>> > David A. Smith
>
> Don , I give you a brief outline of why turning to physicists on this
>matter is a hopeless cause and specifically on the matter of the
>constant axial rotation of the Earth occuring within variable orbital
>motion.
>
>The so-called 'conservation of angular momentum' addresses Kepler's
>second law of planetary ORBITAL motion and has nothing to say about
>the Earth's axial rotation as a seperate motion.

  The earth rotates about its axis, and expresses conservation of
angular momentum as it does so. When the mass is distributed more
closely to the axis of rotation (building ice sheets, for instance),
the rotation rate accelerates. When it is farther away (melting
the ice sheets and letting some of the mass flow towards the equator),
the earth's rotation slows down.

  The earth's motion around the Sun also expresses conservation of
angular momentum. This time including exchanges with the Sun,
Moon, and (primarily) the Jupiter-Saturn system. Nothing new
there.

[snip]

>I have no doubt that many suspect that something has gone badly wrong
>but unfortunately not many at present can put their finger on the
>sourse of the problem.I assure you that it is as simple as the axial
>rotation of the Earth, most people correctly gauge that rotation as 24
>hours exactly (without knowing the principles behind it) while the
>conservation of angular momentum guys have it at 23 hours 56 min 04
>sec.

  Hardly.

  The ancient egyptians defined the 'day' as being 24 hours. It
could equally well have been declared to be 12, or 60, or 10. The
'hour' was not a fundamental unit of time, it was just a way of
subdividing the (solar) day. For some centuries, the hour was not
of fixed length, either, and was different between day and night.
'daytime' was divided into 10 parts, with a warmup and cooldown hour
on either side. Around this time of year, those 'hours' were
pretty long. The nighttime 12 were correspondingly short at this
time of year.

  Since you have some mysterious problem with Newton and other
relatively modern folks, I'll stay away from them.

  We're _given_ that the solar day was _declared_ to be 24 hours.
That was and is true (though we have, now, to deal with the
equation of time, and mean sun vs. actual, for reasons I'll get
to in a minute.

  The Maya had determined the length of the year -- time for the
sun to return to the same place in the sky relative to the fixed
stars -- to be 365.242 (and a bit, but I forget that bit and it
doesn't affect the argument) days. Given the declaration that a
day (specifically solar day) is 24 hours, the year is, then,
8765.808 hours.

  There's a third return time (first = (mean) sun to return to
the meridian, second = sun to return to the same place relative
to the fixed stars) that can be considered, and it's intrinsic
to the other two. That is, time for a fixed start to return to
the meridian. The frequency for that is the sum of the frequencies
of the other two. In terms of period:
1/P3 = 1/P1 + 1/P2
where P1 = mean solar day, P2 = year, P3 = time for fixed star to
cross meridian. P1 = 24, P2 = 8765.808. P3 is, therefore
23.9344696 hours, or 23 hours 56 minutes and 4 seconds.

  This is all strictly on a level of math and observation that
the Maya and many other pre-Newtonian civilizations were carrying
out. All that's needed is to be able to observe the sun and the
stars and to count days.

  Otto Neugebauer's _The Exact Sciences in Antiquity_ has much
more on the above.

  As chronometers of some modest accuracy were developed, folks
discovered that the sun did not move on a steady, accurate, 24 hours
between meridian crossings. Hence the equation of time and
the concept of 'mean sun' were developed. The 'mean sun' moves
at exactly 24 hours between meridian crossings. The real sun can
be up to about 15 minutes ahead or behind that.

  This fact seems to be causing you some angst?

  The motion of the fixed stars (the 23:56:04), however, is a
different matter. That motion shows outstanding constancy.
It's an easy observation, but the constancy is so great that
you need an excellent time piece. Just look for a star crossing
your meridian and time how long it is until it is back in the
same position. The time for this varies by no more than a
few thousandths of a second.

  If the variations in the motion of the sun were due to motion
of the crust, then the stars should also show that motion. Our
observing tools ('til recently) are fixed with respect to the
local crust.

  If it were crustal motions, one might also expect differences
in the motion of fixed stars as observed by people on different
parts of the crust.

  But neither is observed. Everybody sees the same time between
crossings of the fixed stars.

  Note, too, that all the preceding is equally true if you prefer
to consider the rest of the universe to be moving around the earth. 

-- 
Robert Grumbine http://www.radix.net/~bobg/ Science faqs and amateur activities notes and links.
Sagredo (Galileo Galilei) "You present these recondite matters with too much 
evidence and ease; this great facility makes them less appreciated than they 
would be had they been presented in a more abstruse manner." Two New Sciences
Loading