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What Is an Oblate Spheroid?

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  • Written By: Christian Petersen
  • Edited By: Susan Barwick
  • Last Modified Date: 30 September 2014
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An oblate spheroid is a three dimensional solid and is most easily described as a sphere that has been compressed from top to bottom, causing the equator to bulge out. The Earth, and most rotating bodies in space are shaped like this. Forces that act on the Earth as a result of its rotation produce this shape. These forces cause the Earth's mass to try to fly outward as the planet rotates, but gravity holds it together.

An ellipse rotated around its minor axis will describe the three dimensional object known as an oblate spheroid. The ellipse is an oval-shaped two-dimensional construct, defined in geometry as the shape that results from the intersection of a flat plane with a cone. It has two axes: major and minor. A line passing through the center of the ellipse and having its end points positioned the maximum distance apart is the ellipse's major axis and is its maximum possible diameter. The minor axis passes through the center of the ellipse, has endpoints positioned the minimum distance apart, and is the minimum possible diameter of the ellipse.

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This can be visualized by imagining an ellipse so that the longer dimension is horizontal. The minor axis is a vertical line passing through the center of the ellipse connecting the top and bottom. The outline of the ellipse is rigid, and the minor axis is its pivot point. Spinning the ellipse around the minor axis creates an oblate spheroid. The ellipse can be seen as the profile of a planet, with the minor axis as the line through the center of the planet that forms the north and south poles. Numerous mathematical equations exist for describing these types of solids in geometry and trigonometry.

While many people assume that the Earth is a sphere, this is not the case. Of course, variations in the surface features would preclude describing the Earth in terms of any perfect solid shape, but when taking into account the scale of the planet versus the size of its surface features, these shapes can be used as approximations. In reality, the shape of the Earth more closely resembles an oblate spheroid than a sphere. The difference is very small, but the Earth is approximately 42 miles (65 km) wider at the equator than it is from pole to pole. This means that a person standing at either pole is approximately 21 miles (32 km) closer to the Earth's center than someone standing at the equator.

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