Date: Tue Apr 3 12:35:23 2007
Posted By: James Holliday, Grad student, Physics Department, University of California, Davis
Area of science: Earth Sciences
It's true that the Earth's gravity
varies over its surface. While
the average gravitational acceleration at the surface is approximately
9.80 m/s2, it can vary by a factor of around 1%. This
amount is definately measureable, but depending on what you mean,
probably not noticable.
The cause of the gravity
anomalies can be attributed to a combiniation of factors:
- Topography -
Gravity varies by very small amounts depending upon whether you are on
(or even near) hills or hollows.
- Elevation -
Gravity decreases with distance from the Earth, so it will be slightly
less at the top of a high-rise building than at the base. This effect,
however, is reduced slightly at the top of large mountains due to the
increased amount of rock underfoot. Gravity is about 0.2% lower at
the top of Mount
Everest than at sea
Variations - Variations in the density of rocks in the Earth's
lithosphere cause local variations in gravity. Very large
structures such as mountain ranges and ocean basins can lead to
variations of up to 0.05%. Variations due to rock structures related
to oil (such as salt
domes) and metal ore bodies are very small, but can be measured as
anomalies that occur within small geographical areas.
- Latitude -
Gravity is an average 9.78 m/s2 at the equator. It is about
0.5% higher at the poles at an
average 9.83 m/s2. This is due to a combination of effects:
- The spin of the Earth creates an outward force that is
greatest at the equator. The difference is small, but enough to make
the launch of space rockets cheaper near the equator than near the
poles for most intended orbits.
- The Earth's spinning has
distorted its shape, making it about 0.34% fatter at the Equator. The
diameter at the equator is about 12,756 km (7,926 miles). The distance
from pole to pole is about 12,713 km (7,900 miles). This means that
there is more Earth under your feet at the equator, hence an increase
- Earth Tides
- Earth tides, caused by the gravitational attraction of the Sun and
Moon, lead to small variations in gravity around the Earth, depending
on their relative positions in space relative to the Earth.
The GRACE (Gravity
Recovery and Climate Experiment) project has some interesting
pictures of the Earth's gravity field anomalies and a nice
description of how the measurements were made.
I hope this has helped.
James R Holliday
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