February 19, 2009
There are several ways to measure a mountain’s height. While Mt. Everest, on the border of Nepal and China (Tibet province), is the highest mountain in the world above sea level, it’s not the place on the Earth’s surface most distant from the point at the very center of Earth. Due to Earth’s daily rotation on its axis, our planet bulges about 22 km (14 miles) at the Equator. As a result, the distance from the top of Mt. Chimborazo (in Ecuador) to the point at Earth’s center is actually about 2 km greater than to the top of Mt. Everest. In fact, nearby tropical beaches along the Pacific Ocean are actually farther from Earth’s center than is the top of Mt. Everest.
If Earth were a drop of liquid rotating once a day, its shape would be the same. That is, under the long-term centrifugal forces, Earth takes the shape of a sphere slightly flattened at the poles. However, Earth is so close to being a perfect sphere that it appears perfectly round from any point in space. Due to the combined effects of Earth’s oblate shape and centrifugal force, a person who weighs 68 kg (150 lbs) at the North Pole would weigh about half a pound less at the equator. But his or her mass wouldn’t change. This is one reason that scientists generally measure matter by mass rather than by weight, which is dependent on the force of gravity. Furthermore, although Earth has a mass of 6 septillion kg (13 septillion lbs), our planet is weightless in space.