Fig. 67.
As the curvature of the ocean is the same in every direction, it follows that the surface of the ocean is spherical. The same is true of the surface of the land, allowance being made for the various inequalities of the surface. From these and various other indications, we conclude that the earth is a sphere.
56. Size of the Earth.—The size of the earth is ascertained by measuring the length of a degree of a meridian, and multiplying this by three hundred and sixty. This gives the circumference of the earth as about twenty-five thousand miles, and its diameter as about eight thousand miles. We know that the two stations between which we measure are one degree apart when the elevation of the pole at one station is one degree greater than at the other.
57. The Earth Flattened at the Poles.—Degrees on the meridian have been measured in various parts of the earth, and it has been found that they invariably increase in length as we proceed from the equator towards the pole: hence the earth must curve less and less rapidly as we approach the poles; for the less the curvature of a circle, the larger the degrees on it.
Fig. 68.
58. The Earth in Space.—In Fig. 68 we have a view of the earth suspended in space. The side of the earth turned towards the sun is illumined, and the other side is in darkness. As the planet rotates on its axis, successive portions of it will be turned towards the sun. As viewed from a point in space between it and the sun, it will present light and dark portions, which will assume different forms according to the portion which is illumined. These different appearances are shown in Fig. 69.
Fig. 69.