We have all been taught that the earth is a nearly spherical body which every twenty-four hours rotates from west to east around an imaginary line called its axis - this axis having as its extremities the north and south poles respectively - while in the course of a year it completes a revolution around the sun.

To an observer whose view is not obstructed, any part of the earth presents itself as a circular and horizontal expanse, on the circumference of which the heavens appear to rest. Accordingly, in remote antiquity, the earth was regarded as a flat, circular body, floating on the water. But even in antiquity the spherical form of the earth began to be suspected.

It is only on this supposition that we can explain how the horizon of vision grows wider and wider the higher the position we choose, how the tops of towers and mountains at a distance become visible before the bases, how the hull of a ship disappears first as she sails away, and how, as we go from the poles towards the equator, new stars become visible. Besides these proofs, there are many others, such as the circular shadow of the earth seen on the moon during an eclipse, the gradual appearance and disappearance of the sun, and especially the fact that since 1519 the earth has been regularly circumnavigated.

The earth is not, however, an exact sphere, but is very slightly flattened at the poles, so as to have the form known as an oblate spheroid. In this way the polar diameter, or diameter from pole to pole, is shorter than the diameter at right angles to this - the equatorial diameter. The most accurate measurements make the polar diameter about twenty-seven miles less than the equatorial, the equatorial diameter being found to be 7,925.6 miles, and the polar 7,899.14.