Size and Shape
Those involved in the study of measuring the Earth, called geodesists, don't know exactly what the true shape of the Earth is. We often describe it as a sphere, it is more technically called an oblate ellipsoid. That is, the Earth is wider in the middle and flatter at the poles. The equatorial bulge is due to the centrifugal force exerted on the Earth by its rotation. The equatorial diameter Earth is about 7926 miles (12,756 km) while the polar diameter is 7900 miles (12,714 km). The equatorial circumference is approximately 24,900 miles (40,075 km).
Figure 2.10 View of the the Earth taken by NASA's Moderate Resolution Imaging Spectroradiometer aboard the the Terra satellite. (Courtesy NASA JPL. Source) On December 26th, 2004 subduction occurred between the India and Burma plates off the coast of Indonesia making for a slightly more compact Earth. The shift in the Earth's crust resulted in a magnitude 9 earthquake and large tsunami that devastated South Asia. Interestingly, the Earth became slightly more round and the North Pole shifted by about 2.5 cm (an inch) in the direction of 145 degrees East longitude (Science Daily, 2005). From the top of Mt. Everest at 29,029 feet (8848 m) in the Himalayas, to the depths of the Mariana Trench in the Pacific at 6781 feet below sea level, the Earth has a total relief of about 12 miles. Though this seems to be a great distance, it's a mere blip when compared to the diameter of the Earth. Distance on the Earth: Great and Small CirclesWhen we travel, we usually like to take the shortest route between two locations. If you pass a plane through the center of a sphere, the intersection of the plane and the surface of the sphere creates a great circle. Planes passing through any other part of a sphere without going through the center create small circles. An arc of a great circle is the shortest distance between two points on a sphere and therefore is the preferred route for planes traveling great distances, like crossing an ocean. The concept of great and small circles relates to meridians (longitude) and parallels (latitude). Meridians are half of a great circle (180o) whose ends are at the North and South poles. Parallels of latitude are small circles, except for the equator which is a great circle.
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Figure 2.9 Taking readings from a GPS atop the US capitol building
