Shape and Movement of the Earth

Shape of Earth:

Geodesy is the science that studies the shape and size of the Earth. As discussed in Voyagers, the shape of the earth was long known to be round. Aristotle and Pythagoras both argued that the earth was a sphere from the curved shadow it cast during lunar eclipses. About two thousand years later, Sir Isaac Newton suggested that the earth was not a perfect sphere, but rather somewhat flattened at its poles.

From mathematical considerations of the combined gravitational and centrifugal forces which the earth experiences, Newton computed that the Earth's shape should be an oblate spheroid, a solid formed when an ellipse is rotated about its axis (see Figure 3). Expeditions to Peru in 1735 and to Lapland in 1736 confirmed this theory. The difference in axes is about 1 part in 300. This means the earth's equatorial diameter is 7,926 miles, while its polar diameter is 7,900 miles.

Although the difference between equatorial and polar diameters is only 26 miles, the oblate shape of the earth complicates geographical matters.

Local topography also plays a role in the Earth's shape, but on a global scale its role is very small. The largest differences in local topography across the globe are Mount Everest, the highest point above sea level at 29,035 ft. (8,850 m), and the Mariana Trench, the lowest point below sea level at 35,840 ft. (10,924 m). This difference is only a matter of about 12 miles (19 km), which is very minor overall. If equatorial bulge is considered, the world's highest point and the place that is farthest from the Earth's center is the peak of the volcano Chimborazo in Ecuador as it is the highest peak that is nearest the equator. Its elevation is 20,561 ft. (6,267 m).

Movement of the Earth:

The changing seasons are caused by the movements of the Earth. There are two important movements that affect the Earth. The first is the rotation of the Earth around an invisible axis. It takes the Earth about 24 hours to finish one complete...