The earth is round, a sphere – almost. If we look at a picture taken from one of the Apollo missions we cannot tell that the world is not perfectly round.
But the earth is not quite a sphere because it is rotating. This causes the equator to be thrown outward and be further from the center than the poles. Isaac Newton discovered this in 1672, although there was considerable controversy over it for another 100 years.
The effect is small, about 1/3 of a percent (0.0033). On a local scale, say the size of a small country, this cannot be easily measured. However the earth is large. The equatorial radius is 21 km longer than the polar radius. Over long distances the effects add up. In the 1700's the French made a major effort to measure these differences. They did this by measuring the length of an arc on the surface of the earth that was 1 degree in angle. They did this twice, once near the arctic circle and once near the equator. This proved the effects Newton had predicted.
In geodesy and map making the earth is modeled as a flattened ellipse that is rotated on its shorter axis. An ellipse is just a flattened circle. Its equation is very similar to that of a circle.
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