### Re: How much does Earth's surface gravity vary?

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
ID: 1175303471.Es
Message:

Chris,

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 level.
• Density 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:
1. 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.
2. 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 in gravity.
• 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.

Sincerely,

James R Holliday

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