MadSci Network: Astronomy

Re: Does the sperical shape of the Earth change?

Date: Wed May 5 18:25:19 1999
Posted By: Nick Hoffman, Oil and Gas Exploration Geophysics - Melbourne, Australia
Area of science: Astronomy
ID: 921797383.As

Hi, Rosemarie,

You've asked quite an important question there, which leads to a lot of 
different bits of answer. By the time we're finished, you will know a lot 
more about the Earth. 

1) The Earth isn't, in fact, spherical. It nearly is, but not quite. The 
centrifugal "force" generated by the Earth's slow rotation causes the 
equator to bulge just a little bit (about 20km) compared to the poles. 

[note to physics teachers and scientific pedants - I know that centrifugal 
"force" isn't a force at all, but in terms of everyday experience it is an 
easily understood concept. The niceties of inertial pseudo-forces and 
rotating reference frames can come later.]

Superimposed on this ellipsoidal shape are some smaller dents and bulges 
caused by the uneven distribution of mass in the crust and the mantle of 
the Earth. These are typically only a few metres in height, but may extend 
over tens to thousands of km in width.

2)  The shape of the Earth changes on a daily basis! Just as there is a 
tide in the oceans, so also is there a solid body tide within the rock 
beneath your feet. The Earth's surface rises and falls a few centimetres 
every day, under the tidal influence of the Moon and Sun.

3)  The polar spin axis of the Earth is not fixed in space. Instead, the 
Earth slowly precesses like a spinning top, but it takes around 24,000 
years for each slow "wobble".

4)  Over geological time, the continents are not fixed on the Earth's 
surface. They move, along with attached oceanic crust, like conveyor belts 
across the surface of the Earth. 200 Million years ago, America was 
attached to Europe. Things only move a few cm each year, but they still 
move. It is this process (Plate Tectonics) that causes Earthquakes and 
localises many volcanoes in the Pacific "Ring of Fire". Most volcanoes 
world-wide have a link to Plate Tectonics.

5)  The Earth will keep spinning steadily in space for the next 5 billion 
years, without any sudden catastrophic changes. Even the impact of the 
meteor that doomed the Creataceous dinosaurs was like a gnat being squashed 
on the windscreen of a speeding truck. It would take a body as large as the 
Moon to significantly change Earth's spin axis, and the Moon's orbit is 
even more stable than the Earth's spin. 

6) Tides on Earth are only half the story - the same forces also act on the 
Moon and are accelerating it in its orbit. The Moon is steadily receding 
from the Earth, and tidal friction is slowing the Earth's spin and 
increasing the length of day. One day in the far future, the Month and the 
day will be the same length (about 40 days), but by then the Sun will have 
died, so it won't really matter. (and it's so far in the future that by then 
there won't be people - we'll probably have evolved to something else and 
colonised the galaxy).

Do some reading on these topics and amaze your friends with some of the 
strange facts you find.


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