THE EARTH. From the standpoint of the as tronomer, the earth is merely one of the smaller planets of the solar system. Still, since observa tions of the heavens can be made only from its surface, it acquires special importance even from the purely astronomical point of view. It was recognized in the very earliest days of science that a study of the earth's dimensions is funda mental to exact astronomy. When we try to make actual measurements on the sky we are confronted by this remarkable limitation: our instruments enable us to measure the directions of the heavenly bodies, but not their distances.
This obvious fact is seldom set forth with suffi cient emphasis. A knowledge of distances in the heavens can be attained only by calculation from measurements of differences of direction com bined with some terrestrial base-line. To make this base-line as large as possible, it is necessary to use as near as may be an entire diameter of the earth. For this reason the earth has been most carefully measured, and it has been found that its genera] shape is that of a flattened sphere having an average diameter of about 7918 miles, the difference between the greatest and least di ameters being about 27 miles. Yet the very latest researches of modern investigators tend to show that the earth diverges from the regular form of a flattened sphere by measurable amounts. The name geoid has been invented to describe its exact mathematical figure. (See EARTH ; GEO DESY; TRIANGULATION.) The earth revolves on its axis uniformly; and the quantity of time required for one complete revolution has been adopted as the fundamental unit for measuring time in astronomy. This unit
interval is called the sidereal day. (See DAY.) It is of course important in the highest degree that all natural units of measure be absolutely invariable in magnitude. And it has been ascer tained from a comparison of ancient and modern eclipse observations that this necessary condition of constancy is fully satisfied by the day as a unit of time. The axial rotation of the earth is the cause of the sun's, 'noon's, and stars' rising and setting. For as the earth turns on its axis. carrying the observer with it. of course the heaven ly bodies seem to be carried past in the opposite direction. This is much like the apparent spin ning by of houses and fields to an observer in a rapidly moving railway train. These apparent phenomena arising from axial rotation are called diurnal phenomena. In addition to its axial ro tation, the earth travels around the sun once in a year, pursuing, like the other planets, an oval or elliptic orbit in obedience to Kepler's Law. It happens that the gravitational forces controlling the earth's motions are so adjusted that its rota tion axis maintains a nearly constant direction in space. As this axis makes an angle of 23% degrees with the plane containing the annual orbit around the sun, it follows that for any given place on the earth's surface the rotation axis is turned sometimes toward the sun and at other times away from it. This gives rise to the phenomena of the seasons. Of all astronomical occurrences, the diurnal phenomena and the seasons are the ones most closely affecting our daily life.