ASTRONOMY. Astronomy is that branch of science which treats of the heavenly bodies—including practically all the bodies of the universe. The great advance which our times have witnessed in the methods of re search has made it one of the most progressive of the sciences, while it is, at the same time, the oldest of all. The vast extent of its field, including the entire universe within its bounds, leads to its having a number of different branches. There is, first a branch which em braces our general knowledge of the heavenly bodies, their motions, aspects and physical con stitution. This branch is commonly termed descriptive or general astronomy. It is now recognized as having two divisions, one relating principally to the motion, mutual relations and general aspects of the heavenly bodies; the other to their physical constitution, considered individually. The former division is sometimes termed astrometry, because it is principally con cerned with measurements of position, motion, mass, etc. The other branch is termed astro physics, and is that which has received its !greatest development in recent times. There is also a branch which teaches the methods of observing the heavenly bodies, including the in struments used in observation and measure ment, and the principles governing their use, as well as the practical computations incident thereto. This branch is termed practical as tronomy. Another branch is the mathematical one, which determines the orbits and motions of the heavenly bodies by deductive methods, taking as a basis the facts of observation and the laws of motion, especially that of gravita tion. This branch treats of the orbits of the heavenly bodies and of the methods of comput ing the effects of their mutual attraction. It is commonly termed theoretical astronomy, while the more purely mathematical theory is known as celestial mechanics.
The subject of astronomy is treated in the Present work on the following plan: We begin with a brief but comprehensive survey of the universe, referring to special articles — Stars, Universe, Nebula, Solar System, etc., for de tails. This survey will be followed by reviews of Practical Astronomy, Theoretical Astron omy and of the historical development of the science.
Descriptive Astronomy.— Considered as to their nature, the heavenly bodies may be di vided into two great classes; the one, incandes cent bodies which shine by their own light ; the other, opaque bodies which are visible only by reflecting the light of some incandescent body in their neighborhood. Examples of the first class are the stars which stud the heavens at night; examples of the second are the planets, of which our earth is one. From the very na ture of the case, little can be learned of the Possible number of opaque bodies which may exist in the universe. There may be some rather uncertain ground for inferring that they are less massive and less numerous than the incandescent bodies; but it is sometimes sup Posed that they may far outnumber the latter without our being aware of the fact. The stars are scattered through the wilderness of space at distances which baffle all our powers of conception. Light moves with such speed that it would make the circuit of the earth seven times in a single second. But the cases are rather exceptional when a star is so near one of its neighbors that light would not take years to travel over the distance which separates them. Indeed, the only known exceptions be long to the class of double or multiple stars— two or more such bodies forming a system by themselves. There is only one star so near us
that its light would reach us in four years, and the same is probably true of most other stars. That the universe of stars extends to distances which light would require several thousand years to travel is certain; but no well-defined limit has yet been set to its extent. Our sun is one of the stars, and is the one of which we know most because of our proximity to it. It is the centre around which eight great planets and a number of other bodies perform their revolutions. On one of these great planets, the third in the order of distance we dwell. Our knowledge of the heavens is largely con ditioned by our residence on this planet. We see the other planets by the light of the sun, which they reflect. They present to the naked eye the appearance of stars; and it is only when scrutinized with the telescope that they are found to have a measurable apparent size. Vast indeed is their distance from the sun when measured by our standards. Yet, the dimensions of our solar system are very small when compared with the distance which sepa rates the stars. Light passes from the sun to the outer planet, Neptune, in about four hours, while, as we have said, it requires years to reach any star. The nearest star is therefore thousands of times farther than the most distant planet. A most interesting question is whether other stars have systems of planets revolving round them, as our sun has. This is a question which it is impossible to answer conclusively. Such planets would be absolutely invisible through the most powerful telescope that man can ever hope to construct. Yet certain theo retical considerations seem to render it prob able that many, and perhaps the majority, of the stars have planets revolving around them. It is generally held by astronomers that all stars were originally nebula; investigation shows that if the nebula was of a more or less heterogeneous structure it will develop, at least usually, into a central sun attended by a system of planets (see COSMOGONY). As by far the greater part of the nebula of the sky are of a spiral structure, and hence far from having a uniform density, it is indicated that this is the usual condition of these objects; it is thus rendered probable that at least many of the stars are the centres of systems which more or less closely resemble our own. In special cases evidence on the subject is afforded by the spectroscope, which shows that great numbers of stars really have one or more dark bodies revolving around them. But, in order to be observable with the spectroscope, these bodies must be vastly larger than the planets which revolve round our sun. The existence of a planet like that on which we dwell could not be determined even with the best spectroscope. The bodies of the solar system are bound together by the law of gravitation. Were it not for the attraction of the sun each planet would fly off in a straight line through space. Through the attraction of the sun all the planets are kept in their several orbits. Every con sideration leads us to believe that gravitation extends from one star to all the others, but diminishing as the inverse square of the dis tance. Revolving double stars show that sys tems of two stars in proximity to each other are subject to the law of mutual attraction and hence that gravitation is not peculiar to our solar system, but operates throughout the entire visible universe.