Page:Astronomy for Everybody.djvu/131

The globe on which we live, being one of the planets, would be entitled to a place among the heavenly bodies even if it had no other claims on our attention. Insignificant though it is in size when compared with the great bodies of the universe, or even with the four giant planets of our system, it is the largest of the group to which it belongs. Of the rank which it might claim as the abode of man we need not speak.

What is the earth? We may describe it in the most comprehensive way as a globe of matter nearly eight thousand miles in diameter, bound together by the mutual gravitation of its parts. We all know that it is not exactly spherical, but bulges out very slightly at the equator. The problem of determining its exact shape and size is an extremely difficult one, and we cannot say that an entirely satisfactory result is yet reached. The difficulty is obvious enough. There is no way of measuring distances across the great oceans. The measurements are necessarily limited to such islands as are visible from the coasts of the continents or from each other. Of course, the measures cannot be extended to either pole. The size and shape must therefore be inferred from the measures across or along the continents. Owing to the importance of such work, the leading nations have from