cision attainable with the most powerful modern observational machinery. The work of Newton may be said to have been crowned by that of Laplace (1749-1827), who analytically demonstrated the stability of the solar system, and whose Mécanique céleste was a worthy successor to Newton's Principia. Having thus disposed of the mechanism of our solar system, the question suggests itself as to whether this same law of Newton holds sway even to the most distant confines of the visible universe. Does it control the motions of stars whose vast distance allows us barely to glimpse them in our largest telescopes? The answer in the present state of astronomical knowledge is not quite so certain for the distant stellar systems as it is within our immediate neighborhood in space. Yet we may say that observations so far made have brought to light no phenomena positively contradicting the universality of Newton's law.
The Sun. Coming now to a closer inquiry into the details of our subject, we begin with the sun (q.v.), the central body of our own system. (See Solar System.) From a terrestrial point of view, being the source of our light and heat, the sun is certainly the most important of all the heavenly bodies. But considered as a cosmic body the sun is simply a blazing star, and not even a very great one as compared with the average of all the stars. Numerous theories as to its constitution have been advanced. The best authorities now agree that whatever may be the unknown structure of the central mass, the surface layer directly visible to us (the 'photosphere') consists of brightly incandescent matter, partly liquid or even solid, and giving out the sun's 'light. Outside of this is a truly gaseous layer (the 'chromosphere'), very hot, bubbling, boiling, and at times throwing jets of luminous vapor hundreds of thousands of miles into space. Beyond the chromosphere extends another envelope of tenuous matter (the corona, q.v.), so slightly luminous that it can be seen only when a total eclipse shuts out altogether the stronger light of the photosphere. The most conspicuous phenomena upon the photosphere are the sunspots. These are great temporary breaks or openings, sometimes a hundred thousand miles in diameter. They look like the effects of storms or explosions, their cause is not definitely known, but it has been found that magnetic storms on the earth usually occur at periods of great spot-frequency on the sun. No connection with the weather or other terrestrial phenomena has, however, been demonstrated.
Solar chemistry has been studied by means of an instrument called the spectroscope (q.v.) (see Spectroscopy), which enables us to determine the chemical constituents of a luminous body like the sun. The conclusion reached is that the sun contains no chemical elements other than those found on the earth, a fact of great interest in connection with the question of a possible common origin for the whole solar system. One of the most interesting questions in connection with the sun is the probable source of its great and continuing heat. Why does it not become completely consumed as a result of continuous combustion? This question was answered by Helmholtz (q.v.), who showed that the energy required to generate the sun's heat could be derived from a gradual contraction or shrinkage of its bulk. So great is this bulk that a contraction sufficient to produce the heat in question might go on for ten thousand years without producing a diminution in the sun's apparent diameter large enough to be perceived even by our greatest telescopes.
The Earth. From the standpoint of the astronomer, the earth is merely one of the smaller planets of the solar system. Still, since observations 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 fundamental 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 sufficient emphasis. A knowledge of distances in the heavens can be attained only by calculation from measurements of differences of direction combined 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 general shape is that of a flattened sphere having an average diameter of about 7918 miles, the difference between the greatest and least diameters 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; Geodesy; 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 ascertained 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, moon's, and stars' rising and setting. For as the earth turns on its axis, carrying the observer with it. of course the heavenly bodies seem to be carried past in the opposite direction. This is much like the apparent spinning 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 rotation, 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 rotation axis maintains a nearly constant direction in space. As this axis makes an angle of 23½ 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
