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Q: Causes and consequences of variation in Earth's rotation ( Answered 5 out of 5 stars,   0 Comments )
Question  
Subject: Causes and consequences of variation in Earth's rotation
Category: Science > Earth Sciences
Asked by: gottaknow-ga
List Price: $5.00
Posted: 09 Dec 2002 22:42 PST
Expires: 08 Jan 2003 22:42 PST
Question ID: 122261
Does variation in the Earth's rotation affect climate on the short or
long term or have other major consequences for people?  Can activities
like damming, mining & oil drilling redistribute matter in a way that
affects the Earth's rotation significantly and if so how?
Answer  
Subject: Re: Causes and consequences of variation in Earth's rotation
Answered By: kutsavi-ga on 10 Dec 2002 14:30 PST
Rated:5 out of 5 stars
 
Hey there Gottaknow,

The simple answers to most of your questions is “yes.”  

There is a great report done in 1995 at the University of Texas’
Center for Space Research called “Earth System Dynamics: Angular
Momentum and Mass Balance N”.  The report can be accessed here:
http://www.csr.utexas.edu/eos/reports/95anual/
Click on the buttons at the bottom of the pages to navigate the
report.

In the highly detailed, highly technical report, it states that:

“Momentum and mass transport among the atmosphere, oceans, and solid
Earth produce changes in the Earth's rotation, gravity field,
atmospheric circulation, and global sea level. These phenomena, which
are associated with complicated interactions among geophysical,
atmospheric, hydrologic, cryospheric and human-induced processes
within the Earth system, produce observable effects which are being
measured with unprecedented accuracy by pre-Earth Observing System
(EOS) space geodetic sensors.”

What that means is that there are many things that influence the
rotation of the Earth producing complicated interactions with the
atmosphere, ice sheets, water and inhabitants, including humans, of
the planet.

Basic to your question is the fact that the Earth’s orbit around the
sun is not a perfect circle, and thus the Earth is closer to the sun
at different times of the year.  In addition, there is variation in
how close the Earth gets to the Sun in any given year.  The effects
produced by these variations are grouped under something called the
Milankovitch Theory:

“The Milankovitch theory suggests that normal cyclical variations in
three of the Earth's orbital characteristics is probably responsible
for some past climatic change. The basic idea behind this theory
assumes that over time these three cyclic events vary the amount of
solar radiation that is received on the Earth's surface.

The first cyclical variation, known as eccentricity, controls the
shape of the Earth's orbit around the sun. The orbit gradually changes
from being elliptical to being nearly circular and then back to
elliptical in a period of about 100,000 years. The greater the
eccentricity of the orbit (i.e., the more elliptical it is), the
greater the variation in solar energy received at the top of the
atmosphere between the Earth's closest (perihelion) and farthest
(aphelion) approach to the sun. Currently, the Earth is experiencing a
period of low eccentricity. The difference in the Earth's distance
from the sun between perihelion and aphelion (which is only about 3 %)
is responsible for approximately a 7 % variation in the amount of
solar energy received at the top of the atmosphere. When the
difference in this distance is at its maximum (9 %), the difference in
solar energy received is about 20 %.

The second cyclical variation results from the fact that, as the Earth
rotates on its polar axis, it wobbles like a spinning top changing the
orbital timing of the equinoxes and solstices. This effect is known as
the precession of the equinox. The precession of the equinox has a
cycle of approximately 23,000 years. According to illustration A, the
Earth is closer to the sun in January (perihelion) and farther away in
July (aphelion) at the present time. Because of precession, the
reverse will be true in 11,500 years and the Earth will then be closer
to the sun in July. This means, of course, that if everything else
remains constant, 11,500 years from now seasonal variations in the
Northern Hemisphere should be greater than at present (colder winters
and warmer summers) because of the closer proximity of the Earth to
the sun.”
From:  
http://www.geog.ouc.bc.ca/physgeog/contents/7y.html

A main factor contributing to the Milankovitch Theory is “precession”,
or the “wobble” of the Earth’s axis.  Excellent information on how
this affects Milankovitch is found in a paper from the US Naval
Observatory titled “The Seasons and the Earth’s Orbit – Milankovitch
Cycles”.  Here is a small snippet from that report:

“Most of the difference in the average lengths of the two kinds of
year is due to the very slight change in the direction of the Earth's
rotation axis in space from one year to another. We usually think of
the Earth's axis as being fixed in direction - after all, it always
seems to point toward Polaris, the North Star. But the direction is
not quite constant: the axis does move, at a rate of a little more
than a half-degree per century. So Polaris has not always been, and
will not always be, the pole star. For example, when the pyramids were
built, around 2500 BCE, the pole was near the star Thuban (Alpha
Draconis). This gradual change in the direction of the Earth's axis,
called precession, is caused by gravitational torques exerted by the
Moon and Sun on the spinning, slightly oblate Earth.

Because the direction of the Earth's axis determines when the seasons
will occur, precession will cause a particular season (for example,
northern hemisphere winter) to occur at a slightly different place in
the Earth's orbit from year to year. At the same time, the orbit
itself is subject to small changes, called perturbations. The Earth's
orbit is an ellipse, and there is a slow change in its orientation,
which gradually shifts the point of perihelion in space. The two
effects - the precession of the axis and the change in the orbit's
orientation - work together to shift the seasons with respect to
perihelion. Thus, since we use a calendar year that is aligned to the
occurrence of the seasons, the date of perihelion gradually regresses
through the year. It takes 21,000 years to make a complete cycle of
dates.”

Activities like damming, mining & oil drilling, at this point in time,
are not capable of redistributing enough mass to cause any difference
in rotation or orbit.  That level of redistribution can be done
tectonically, however.  Plate Tectonics, or continental drift,
redistributes the Earth’s landmasses, and might affect rotation,
although I could not readily find any solid information on this on the
web.  Man can however affect climate change by producing changes in
the atmosphere that could accelerate or enhance rotationally produced
climate change.

Hope this answers your interesting question!  Let me know if you need
clarification of any of the points.

Kutsavi

SEARCH TERMS

earth rotation variation climate change
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effects of tectonics on earth rotation
://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=utf-8&q=effects+of+tectonics+on+earth+rotation&btnG=Google+Search

Clarification of Answer by kutsavi-ga on 10 Dec 2002 14:35 PST
Regarding precession and the article I quoted from in my answer, I
neglected to give the link to the article. So sorry, here is the URL:
http://aa.usno.navy.mil/faq/docs/seasons_orbit.html

Request for Answer Clarification by gottaknow-ga on 10 Dec 2002 20:42 PST
Many thanks for a quick and thorough answer.  I was unclear as to the
consequences for people, however.  The time scales addressed in your
answer (23000 years, 100000 years) suggest that the variation is
gradual - and perhaps predictable  - nothing that would be noted
within the lifetime of any given person, or "chaotic."  Is there any
indication that the wobbling or orbital changes affect people in a way
that they might be able to perceive or measure on a day to day basis -
or maybe a year to year basis?  The Texas report - while surely
comprehensive, is in jargon that is way beyond me.  Someone suggested
to me that the human activities that redistribute mass (damming mining
etc) would actually have implications for global warming - i.e. on a
short time scale.  That does nto seem intuitive to me - the time scale
of the vents in these reports are more coarse grained.

Many thanks for your help in "translating" :)

Cheers,
Deb

Clarification of Answer by kutsavi-ga on 11 Dec 2002 08:02 PST
Hi Deb, 

Climate change is a slow, systemic thing.  It is not a sudden, chaotic
occurrence wreaking havoc on the planet.  The worry is that OVER TIME
the changes brought about by climate change could be devastating. 
There is a web site with lots of good articles on the effects of
climate change at

http://www.changingclimate.org  

Some titles of reports on the site are:

Food in a Climate of Disparity, (3 Part Series)
Climate Change; Spiritual Crisis
Counting the costs of UK Climate Change - and hte benefits
Lighting up an Island
The Impacts of a Higher Carbon Future
Climate Change and Food
Rapid warming in Antarctica
Climate Change and Extreme Weather
Climate Change Hits Your Pocket
Soil Matters
Health Ups and Downs of Climate Change
Death in the Marshes?
Climate Change and Human Health in Europe

You also ask “Is there any indication that the wobbling or orbital
changes affect people in a way that they might be able to perceive or
measure on a day to day basis - or maybe a year to year basis?”  Well,
the answer to that would have to be a firmly stated “Well….”

Neither example I’m going to provide you gives evidence that people
are AFFECTED in any way by the mere rotation of the Earth.  They are
not.  And people cannot affect the rotation of the Earth by their
activities.  (More on that below.)  The examples I’m using below only
show the mechanism that has been at work for millions of years, and
that continues to work.

As far as your saying “Someone suggested to me that the human
activities that redistribute mass (damming mining etc) would actually
have implications for global warming - i.e. on a short time scale.”, I
would have to take exception and make sure that you understand that
human changes can bring about climate change, but NOT through the
redistribution of mass on the earth by mining and damming.  These
things take place on such a thin layer of the already thin crust, that
the mass simply IS NOT moved that could create anomalies in the
Earth’s rotation.  Human’s effects on climate happen on the chemical
side of things within the atmosphere and the water cycle.

Your question, however, brings to mind Two things:  The “analemma”,
and a certain canyon in the desert southwest.  First let’s see what
the analemma is.  The following is from the web site
http://www.analemma.com/Pages/framesPage.html
________________________________________
“Have you ever seen this figure-8 on a globe and wondered what it is?
It is simply this: if you could record the position of the sun in the
sky at the same time every day, let’s say sometime around noon and
subtracting one hour if you are observing daylight saving time, you
would notice that the sun takes a rather strange path. You might
notice that at certain times throughout the year the sun's position
not only varies higher and lower (North and South) as you would expect
with the change of the seasons, but also slightly east and west. This
figure-8 path that the sun makes in the sky is called the analemma. On
some days you might notice that the sun is not in the sky where,
according to the time on your watch, you would expect it to be…Why
does the sun take this strange path? There are two reasons and they
are completely independent from each other:

1. The Earth is tilted on its axis 23.5 in relation to the plane of
its orbit around the sun.

2. The Earth does not orbit the sun in a circle, but in an ellipse.

It is simply the sum of these two effects that causes the analemma.”
_______________________________________
This web site gives information on how to create your own analemma by
observing the sun on a daily basis, but it will take an entire year of
making observations to complete the project.  When it is completed,
however, you will have the perfect representation of the earth’s orbit
around the sun as seen from your particular latitude.

Speaking of analemmas, an interesting one was seen in the movie
Castaway.  In his cave after he’d been on the island for a couple of
years, there is a quick scene that shows his calendar and how he has
kept track of the days.  Drawn there, supposedly by marking where the
sun fell through the hole in his cave on certain days, is a perfect
analemma.  Check it out the next time you see the movie.

Now, what about that canyon?  The Anazazi, “The Ancient Ones,” who may
be the ancestors of today’s Hopi Indians in the southwest deserts of
New Mexico and Arizona, constructed an elaborate celestial observatory
back around the 1300’s in Chaco Canyon, in northwestern New Mexico.

The site is known for its many solstice markers; rocks whose shadows
fall in the exact center of etched circles on the longest and shortest
days of the year.

There is also a lunar alignment at Chaco that is very interesting. 
(There is a lunar cycle that represents its eccentric orbit around the
Earth, also.)  The moon will seem to rise further and further to the
north or south for a period of just over 8 years.  There is then an
apparent standstill for a night or two, then the moon will begin it’s
8 – 9 year cycle all over again.

The archaeoastronomers at Chaco not only traced the path of the sun
during the solstices perfectly, they also constructed walls and
alignments that have been proven to follow the lunar “stand still”
cycle of 18 years.

The ruins at Chaco still exist, and their alignments have been
scrutinized and surveyed by any number of professional archaeologists,
and all have come up with the same findings. Now, when you hear about
these walls and alignments, don’t think of one small structure.  In
the surrounding area, the extent of the Chaco observatory has been
estimated to have covered 25 square miles and comprised at least 5
very large, still very present, (if in ruins), buildings.  A fairly
accessible article on the present-day archaeology being done at Chaco
National Historic Culture Park, managed by the U. S. Park Service, is
here:
http://www.solsticeproject.org/lunarmark.htm

I’m interested in using this example to show the continuity and
essential stability of the Earth’s rotation over time.  The alignments
built 700 years ago at Chaco Canyon still work!  These alignments and
petroglyphs still line up on the longest and shortest days of the
year, and even the 18 year lunar cycle calendar is still useable.  In
my mind, this speaks wonders for the ongoing stability of the Earth’s
rotation and orbit, and doesn’t provide much fodder for those who
claim that mining and damming interests have had effects on the
Earth’s rotation.

The effects that man has on climate change do not have anything to do
with the rotational stability of the Earth.  Man’s changes are done
mainly in the atmosphere via gasses and pollution, and to the water
cycle of the planet through these same means.

Ok Deb, I hope this goes a ways toward answering your question.  If I
can clarify anything else, don’t hesitate to let me know.

Kutsavi
gottaknow-ga rated this answer:5 out of 5 stars and gave an additional tip of: $10.00
This was my first time using Google Answers and I am delighted! 
Kutsavi provided answers in logical bits, and then gave examples and
links that helped me see that information in a larger context.  I hope
you are an educator, Kutsavi, because you clearly have an ability to
communicate complex scientific ideas in language that people can
understand.  I appreciate your time and effort in providing an
exceptional answer :)

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