The Planet Earth

FOUR planets in the solar system are smaller than the planet earth ; four are considerably larger. The earth, therefore, is not an outstanding member of the vast solar family. However it is the most important of all the planets to you and me, for it is our home in space  the vantage point from which we view the universe. Many theories have been proposed to explain the origin of the earth and the other planets. We have discussed some of them in other articles . They are little more than ingenious conjectures, for they are based on insufficient data. However, though we know little about the beginnings of our planet, we know a great deal about its shape, structure, properties and motions. We are quite certain that the earth is round or nearly so. Photographs of the earth taken from rockets soaring far above its surface show distinctly the curvature of our planet. Before this factual evidence became available, we could infer the roundness of the earth from certain facts. It was known, for example, that the hull of a receding ship, following the curve of the earth, disappears from view before its superstructure does. It was known also that as a total lunar eclipse develops, the earth casts a curved shadow on the moon. The earth is not a perfect sphere. Technically speaking, it is an oblate spheroid, or flattened sphere. Our planet bulges somewhat at the equator, because of its rotation. It is believed that while the earth was still in a molten state, its spin caused particles to be forced outward from the . axis of rotation. The particles were piled up least of all near the poles and most of all at the equator. Later a solid crust formed at the surface of the sphere, which had become slightly distorted. 

The diameter of the earth is 7,900 miles from pole to pole, and it is 7,927 miles in the equatorial zone. The difference between the diameters is comparatively slight, therefore  it comes to a little more than one part in 300. If the earth were represented by a globe 18 inches in diameter, the polar radius would be only 1/32 of an inch less than the equatorial radius. Someone has said that the earth is more nearly a sphere than most of the balls you would find in a bowling alley.