Regents Prep: Earth Science: Introduction:
Oblate Spheroid

The Earth's True Shape
The Earth is not a perfect sphere. Due to it's rotation, the Earth (like all rotating planets) has a slightly distorted shape. The rotational momentum tends to force the matter to bunch up in the middle. In the case of the Earth, this "middle" is the equator.

Oblate Spheroid
The true shape of the Earth called an Oblate Spheroid. The term "Oblate" refers to it's slightly oblong appearance. The term "Spheroid" means that it is almost a sphere, but not quite. One of the most important things to remember about the Earth's shape is that it is only very slightly oblate. The diameter from the North Pole to the South Pole (the shortest diameter) is approximately 12,714 km. The equatorial diameter (the longest diameter) is approximately 12,756 km. This is not a big difference, but it does make the Earth not quite a sphere.

Evidence of the Earth's True Shape
There is a substantial amount of evidence which supports the conclusion that the earth is a slightly oblate spheroid. Three examples of such evidence are:

  • Most obviously, pictures taken from space provide evidence of the Earth's shape. The Earth is so close to being a perfect sphere that when viewed from any point in space the Earth appears spherical. If accurate measurements are performed, however, it can be shown that the Earth is not quite a perfect circle.
  • Very accurate measurements of the positions of the stars also provide evidence of the Earth's shape. The position of the stars (and Sun) appear to change as we move over great distances on the earth's surface. The most notable example of this involves the changing positions of the North Star (Polaris).

Precise gravity measurements also support the conclusion that the Earth is not perfectly spherical. The pull of gravity on an object changes as the distance of the object from the center of the earth changes. The further an object gets from the center of the Earth, the less it weighs. If the Earth were a perfect sphere, then objects would weigh exactly the same any place on Earth (as long as they were at the same elevation). In reality, the weight of an object varies as it changes it's position on the Earth's surface. At the equator, where the Earth's diameter is greatest, objects weigh a little less. At the Poles, where the Earth's diameter is the least, objects weight a little more.

The Earth's surface has many different elevations. The changing elevations of the land (mountains, canyons, etc.) are called relief. Even though these features may seem quite impressive to us, they are insignificant when compared with the size of the Earth. Even the tallest mountain, Mount Everest, has a height that is only about .07% the diameter of the Earth! The bottom line is that the Earth is really very smooth.

Models of the Earth
Models are representations of objects that aid in our understanding. If we were to choose a scale model of the Earth, what objects would make a good miniature Earth? We might be tempted to choose an oblate object to show the true shape, but that would be inaccurate! Since the Earth is so slightly oblate, and the Earth's relief is so insignificant in comparison with its size, the best scale models of the Earth would actually be very round and very smooth. Billiard balls, marbles, ping pong balls, and other smooth spheres are the best representation of the Earth's true appearance.


Created by Thomas Elkins
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