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The Shape of the Earth - The earth is the shape of a ball that is flattened at the poles and bulged at the equator, so that the equatorial diameter is nearly 27 miles longer than the polar diameter. It approaches an oblate spheroid more nearly than any other mathematical figure. Evidence that the earth has a curved and not a flat surface:
  • Its shadow on the moon is always a curved one. Could this be true of a flat surface? 
  • New stars appear in front of the observer and old ones disappear behind him as he travels toward the north or south. How would it be on a flat surface? 
  • The horizon expands rapidly as the observer ascends to higher altitudes. Would this be true on a flat surface? 
  • At sea the slender toprigging of a vessel is visible farther than the larger but lower hull. Why? 
  • There is a marked difference in time with a change of longitude; thus the sun rises more than three hours later in San Francisco than it does in Philadelphia, and nearly nine hours later than it does at London. How would it be if the earth were flat? 
  • The earth has been circumnavigated many times. 
  • The flattening at the poles is indicated by the increased weight of a body in high latitudes over that of the same body at the equator, and by the greater length of a degree of latitude near the poles.
Cause of the Shape of the Earth - Nearly 200 years ago it was shown that an oblate spheroid is one of the figures of equilibrium for a rotating body, and the degree of oblateness or flattening is due to the rate of rotation. More recently it has been shown that the oblateness of the earth corresponds to the requirements of a rotating fluid mass of the size and rate of rotation of the earth.

This was cited as evidence that the earth was fluid at one time in its history before reaching its present solid form. But there are good reasons for thinking that a solid earth would take the same shape after a long period of time, due to the shifting of materials on the surface, or according to the planetesimal hypothesis there would be more material accumulate at the equator than at the poles.

Gravitation shapes the material into a sphere. Rotation causes the flattening of the sphere into the oblate spheroid. It is gravitation that holds bodies on the earth, and the force increases with the mass of the planet. If the earth were the size of the moon, bodies would have much less weight on its surface.

Gases would be so light that they would fly off into space and there would be no atmosphere, hence no water, and no life. If the earth were as large as Jupiter, bodies on the surface would be correspondingly heavier.
Physical geography


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