Oblate and Prolate Spheroid
The shape of the earth is that of a round ball or sphere slightly flattened at two opposite sides. Such a body is termed a spheroid. There are two kinds of spheroids-oblate and prolate; the former as the shape of an orange, the latter that of a lemon. The straight line that runs through the centre of a sphere or spheroid and terminates at the circumference is called the diameter. If the sphere rotates-that is, moves around like a top-the diameter on which it turns is called the axis. In the oblate spheroid the axis is the shorter diameter; in the prolate spheroid the axis is the longer diameter.