Earth’s Shape, Gravity and the Geoid


It is rather ironic that satellites in space have revealed the true shaped of the Earth! One can no longer casually describe the Earth as a football. The shape of the Earth is now described by the geoid, which is an imaginary topographical surface where gravity is constant at every point.

One can think of the geoid as the mean sea level governed by gravity without the effects of winds, tides and currents. The imaginary surface defines the horizontal everywhere and reflects variations in the gravity field. The gravity signal, which changes minute by minute, has given a new insight into the interior of the Earth. The height of the geoid varies by as much as 100 meters from the highest to the lowest point. For example, a swimmer, off Kanyakumari, is 200 meters closer to the center of the Earth than another swimmer near Borneo.


Today very accurate gravity monitors are in Earth orbit. Called GRACE (for Gravity Recovery and Climate Experiment), the mission has two satellites in a 500 km polar orbit. A unique feature of the mission is that they go around the Earth 220 km apart and as one is pulled closer to the Earth at places by increased gravity field of that part of the Earth, the other satellite measures the distance between the two satellites to the nearest micrometer and the maps are 100 times more accurate than the best available! The satellite sends the data five times a day to ground controllers. In the first 30 days, the satellite has exceeded the data collected in the over 30years of previous studies. As a result, scientists today are in a better position to understand the movement of tectonic plates that trigger earthquakes. The accuracy can further be improved.

The more accurate one can determine the geoid, the more accurate would be the Global Positioning System (GPS) used the world over for obtaining one’s exact location on the Earth in terms of latitude, longitude and altitude. The gravity map will indicate the changing mass of polar ice caps, the varying mass of water resources on land, changes in the sea temperature as well as a better profile of the atmosphere. The data would help in tracking global changes in soil atmosphere and throw light on the Earth’s molten rock beneath the crust. Accurate measurement of gravity is also useful for launching long-range missiles as well as in the search for oil.

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