ASTRONOMY. CHAPTER I. METHODS OF INVESTIGATION AND MEANS OF OBSERVATION. 1. The solar system.—The earth, which in the economy of the universe has become the habitation of the races of men, is a globular mass of matter, and one of an assemblage of bodies of like form and analogous magnitude which revolve in paths nearly circular round a common centre, in which the sun, a globe having dimensions vastly greater than all the others, is established, maintaining physical order among them by his predominant attraction, and ministering to the well-being of the tribes which inhabit them by a fit and regulated supply of light and heat. This group of bodies is the SOLAR SYSTEM. which 2. The stellar universe. In the vast regions of space surround this system other bodies similar to the sun are placed, countless in number, and most of them, according to all probability, superior in magnitude and splendour to that luminary. With these bodies, which seem to be scattered throughout the depths of immensity without any discoverable limit, we acquire some acquaintance by the mere powers of natural vision, aided by those of the understanding; but this knowledge has received, especially in modern times, prodigious extension from the augmented range given to the eye by the telescope, and by the great advances which have been made in mathematical science, which may be considered as conferring upon the mind the same sort of enlarged power as the telescope has conferred upon the eye. -- 3. Subject of astronomy-origin of the name. -The investigation of the magnitudes, distances, motions, local arrangements, and, so far as it can be ascertained, the physical condition of these great bodies composing the UNIVERSE, constitutes the subject of that branch of science called ASTRONOMY, a term derived from the Greek words aпp (aster), A STAR (under which all the heavenly bodies were included), and voog (nomos), A LAW-the science which expounds the LAWs which govern the motions of the STARS. 4. It treats of inaccessible objects. It is evident, therefore, that astronomy is distinguished from all other divisions of natural science by this peculiarity, that the bodies which are the subjects of observation and inquiry are all of them INACCESSIBLE. Even the earth itself, which the astronomer regards as a celestial object an αστην, is to him, in a certain sense, even more inaccessible than the others; for the very fact of his place of observation being confined strictly to its surface, an insignificant part of which alone can be observed by him at any one moment, renders it impossible for him to examine, by direct observation, the earth AS A WHOLE,— the only way in which he desires to consider it,- and obliges him to resort to a variety of indirect expedients to acquire that knowledge of its dimensions, form, and motions which, with regard to other and more distant objects, results from direct and immediate observation. This circumstance of having to deal exclusively with inaccessible objects has obliged the astronomer to invent peculiar modes of reasoning, and peculiar instruments of observation, adapted to the solution of such problems, and to the discovery of the necessary data. 5. Direction and bearing of visible objects.— The eye estimates only the direction or relative bearings of objects within the range of vision, but supplies no direct means of determining their distances from each other, or from the eye itself. The absolute direction of a visible object is that of a straight line drawn from the eye to the object. The relative direction or bearing of an object is determined by the angle formed by the absolute direction with some other fixed or known direction, such as that of a line drawn to the north, south, east, or west. 6. They supply the means of ascertaining the distances and positions of inaccessible objects. By comparing the relative bearings of inaccessible objects, taken from two or more accessible points whose distance from each other is known, or can be ascertained by actual measurement, the distances of such inaccessible objects from the accessible objects, from the observer, and from each other, may be determined by computation. Such distances being once known, become the data by which the mutual distances of other inaccessible objects from the former, and from each other, may be in like manner computed; so that, by starting in this manner from two objects whose mutual distance can be actually measured, we may proceed, by a chain of computations, to |