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Shape of the Earth: The Oblate Spheroid

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Earth Shape Oblate Spheroid

What is the shape of the Earth?

Earth is not a perfect sphere. Its shape is an oblate spheroid. This just means that it flattens at the poles and widens out at the equator.

Earth bulges at the equator because of the centrifugal force during rotation.

Like spinning a pizza, the mass pushes outwards and flattens out along the axis of rotation.

Earth radius as a datum

Geographers model Earth’s shape as an ellipsoid, which is a sphere slightly flattened at the poles. They use a datum to reference geographic coordinates on Earth.

A datum describes the shape of the Earth in mathematical terms. It defines the radius, inverse flattening, semi-major axis, and semi-minor axis for an ellipsoid.

For example, the WGS84 datum identifies the longest diameter of an ellipse (semi-major axis) as 6,378,137.0 m. Next, the semi-minor axis is 6,356,752.3 m.

So, this aligns well with the radius of Earth at the equator as 6,378 km. Finally, the squashing at the poles is slightly less at about 6,357 km.

Why Mount Chimborazo is higher than Mount Everest

Earth Bulge

If you stand at mean sea level at the equator, you would be 21.4 km farther away from the center of Earth compared to standing at the north and south pole.

This is because the equator bulges and forms an oblate spheroid. So if you measure from the center point of Earth outwards, Mount Chimborazo is the highest point on Earth because of the equator bulge height advantage.

But if you compared height above mean sea level, then Mount Everest is the tallest mountain. While Mount Chimborazo has a peak elevation of 6,263 m, Mount Everest towers at 8,848 m tall.

Shape of the Earth: The Oblate Spheroid

The shape of the Earth is an oblate spheroid, meaning that its two bulges are slightly different sizes when you compare the equator to the poles.

Do you want to learn more about the Earth and other concepts of Earth Science? If so, there are a lot of career opportunities available as well. These online courses can help.

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3 Comments

  1. There’s only a 0.3% difference. as BD said, do you REALLY think you’ll notice that?

  2. There’s only a 0.3% difference.. Of course you won’t see that difference in a picture.

    If the earth had a radius of 10cm at the poles it would have a radius of 10,03cm at the equator. That’s a 0.3mm increase, you would not notice that difference unless you measured it with precision tools.

    Let’s look at another analogy. If you have one dude that’s 175cm tall and another that’s 0.3% taller he would only be 175.5cm tall. Would you be able to tell which one of them were taller in a picture? No, you would not.

  3. But Every. Single. Damn. Photo. taken from space by astronauts shows a perfectly spherical Earth. Every single one. Those photos are not faked as flat earthers like to say – they are real. The oblate Earth theory is therefore known to be false.

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