MadSci Network: Physics

Re: Does constructio affect in any way earth rotation

Date: Mon Aug 7 18:44:58 2000
Posted By: Michael Costello, , Chemistry, Physics, and Statistics, 3D Systems
Area of science: Physics
ID: 964541090.Ph

Earth’s Irregular Rotation

	The rotation of the Earth is undergoing constant change.  The 
speed at which it turns varies from day-to-day, and the orientation of the 
axis around which it turns varies from day-to-day.  The Earth is like a 
ball spinning around an imaginary stick.  The stick slowly wobbles a 
little, and this changes the way the Earth rotates.  Things are also 
happening to the ball that change the speed at which it rotates [ref. 1].  
Not all of the things that effect the way the Earth rotates are well 
understood.  One of the things which effects the way the Earth rotates is 
construction.  The effect of construction, however, is small compared to 
the other things that change the Earth’s rotation.  Two of the 
organizations that monitor the changes in the Earth’s rotation are the 
NEOS (National Earth Orientation Service) and the IERS (International 
Earth Rotation Service).
	To understand the way the Earth’s rotation changes, it is 
important to know a little about the structure of the Earth.  A simple 
model of the Earth has four layers.  The outer layer is called the crust.  
It is made up of relatively solid rock and has very liquid oceans pooled 
in some of its low points.  The next layer is the mantle.  It is made up 
of relatively solid rock on the top and partly molten rock on the bottom.  
The next layer is the liquid outer core.  It is made up of liquid metal.  
The last layer is the solid inner core.  It is made up of solid metal.  It 
is believed that the metals are nickel and iron.  Surrounding all this, of 
course, is the Earth’s atmosphere—the air we breathe.
	These layers are undergoing constant changes in the way they 
move.  This leads to constant changes in the distribution of mass on the 
planet.  Anytime mass on Earth moves, the rotation of the Earth changes.  
Some movements of mass that effect the Earth’s rotation are seasonal 
wind patters and ocean currents, the tides in the oceans, large 
earthquakes, pumping of ground water, construction of reservoirs, and the 
freezing and melting of glaciers.  Some of the bigger contributors to 
these mass movements in the Earth are the gravitational effects of the 
Sun, Moon, and planets and seismic activity in the Earth [refs 1, 2, &3].
	The impact of all the above events (and others not listed) on the 
rotation of the Earth is not perfectly understood.  The time it takes for 
the Earth to undergo one complete rotation (one day) is slowing down 
because of tidal friction.  The gravitational pull of the moon causes the 
water in the oceans to slosh around.  This sloshing is slowing down the 
Earth.  The average length of a day is increasing from 0.0015 seconds to 
0.0020 seconds each century because of tidal friction [ref. 1].  The 
length of today will probably not be the same as the length of yesterday.  
The day-to-day variation in the length of a day can be as much as 0.00205 
seconds because of changes in the direction and speed of air currents and 
the gravitational effects of the Sun and Moon on the Earth’s liquid core 
[ref. 1].  These changes are too small for a person to notice by observing 
day-to-day changes with a wristwatch or by comparing the time of sunrise 
and sunset to our daily schedules.  But these are some of the biggest 
changes in the Earth’s rotation.
	The construction of cities certainly can change the way the Earth 
rotates.  However, the amount of mass moved for the construction of even a 
very large city is miniscule compared to the amount of mass moved by air 
currents and ocean tides.  The changes in the Earth’s rotation caused by 
changes in large masses like the atmosphere and oceans cannot be perceived 
without very expensive and sensitive equipment.  So we cannot expect to 
perceive the changes in the Earth’s rotation caused by construction, but 
we do know that small changes are caused by construction.  With careful 
observation and study, we may even someday be able to predict the impact 
on Earth’s rotation of building a skyscraper.


Ref. 1
Ref. 2
Ref. 3 http://ilrs.gsfc.nasa.

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