Brahé. -Halley.—Hevelius.- Flamsteed.-Modern Catalogues.-Bradley.—La- caille.-Mayer.-Maskelyne.-Publication of the Histoire Céleste of Lalande.- Piazzi.-Groombridge.-Zone Catalogues of Stars.-Bessel.-Argelander.—San- tini.—Catalogue of the Astronomical Society.—Catalogues of Southern Stars.— Fallows. Brisbane.-Johnson.-Henderson.-Standard Catalogues of Stars.- Catalogue of the British Association.—Recent Standard Catalogues . Early Notions of the Telescope.—Invention of the Telescope in Holland. - Galileo constructs a Telescope. - Kepler proposes the Telescope composed of Two Convex Lenses.-This Instrument first applied to Astronomical Purposes by Gascoigne.-Telescopic Observations of Huyghens and Cassini.-Reflecting Telescope proposed by Gregory.-Newton executes a Reflecting Telescope.— Efforts of his Successors to construct these Instruments.-Invention of the Achro- matic Telescope by Dollond.-Reflecting Telescopes executed by Herschel.- Modern Improvements in the Refracting Telescope.-Improvements in the Con- Origin of Stellar Astronomy.-Physical Changes observed in the Starry Regions. -Disappearance of Stars from the Heavens.-New Stars.-Stars of Variable Lustre - Photometric Researches on the Stars.-Attempts to determine their Apparent Diameters.-Space-penetrating Power of Telescopes.-Applied to ascertain the relative Distances of the Stars. -Absolute Distances of the Stars determined by Photometric Principles.-Parallax of the Fixed Stars.-Early Researches on the Subject. - Modern Researches. - Bessel.—Henderson.— Struve.-Peters.-Proper Motions of the Stars.-Motion of the Solar System in Space. Double Stars.-Discovery of their Physical Connexion by Sir William Herschel.—Methods for determining the Elements of their Orbits.—Nebula.— Speculations of Sir William Herschel.-Modern Researches on the Subject.— Sir John Herschel.-The Earl of Rosse.-Early Speculations on the Milky Way. -Theory of Wright.—Observations of Sir William Herschel.-Speculations of that Astronomer on the breaking up of the Milky Way.-Researches of Struve on the Distribution of the Stars in Space.-Gauges of Sir John Herschel in the Southern Hemisphere.-Speculations of M. Struve on the Extinction of . V.-Note respecting Horrocks. VI.-Account of some recent Results of Astronomical Observation VII.-Copy of the Observation of y Draconis which originally suggested to Bradley HISTORY OF PHYSICAL ASTRONOMY. INTRODUCTION. ASTRONOMY is not only one of the most ancient of the physical sciences, but also one of those which present the most alluring invitations to the contemplative mind. The starry heavens, spangling with countless luminaries of every shade of brilliancy, and revolving in eternal harmony round the earth, constitute one of the most imposing spectacles which nature offers to our observation. The waning of the placid moon, the variety and splendour of the constellations, and the dazzling lustre of the morning and evening star, must in all ages have excited emotions of admiration and delight. Sometimes the occurrence of an eclipse, or the sudden appearance of a comet, would create universal astonishment and terror; for these unusual phenomena have been generally regarded in early times as manifestations of divine displeasure, and the precursors of some impending calamity. But the wants of mankind rendered indispensable some degree of attention to the appearance of the heavens, even in the rudest state of society. The sun and moon minister so obviously to our subsistence and comfort, that their motions could not fail to be watched with interest in all ages. The stars, too, would soon be found to subserve some useful purposes. The mariner would find in them an unerring guide, while pursuing his way through the ocean; and the husbandman, by observing the times of their rising and setting throughout the year, would obtain indications of the change of the seasons, and would thereby be enabled to regulate the labours of the field. But a powerful incentive to study Astronomy in early ages originated in a delusive opinion, that the destinies of human life were affected by the aspects and positions of the stars. Nor is it to be wondered at that these unapproachable objects should have been invested with a mysterious influence before science had disclosed their real nature. If the sun, by advancing with majestic regularity in his annual course, exercised so benign an influence on the animal and vegetable world, the planets, on the other hand, by their wayward evolutions and ever varying configurations, appeared, naturally enough to minds imbued with imperfect notions of the purposes of creation, meetly to foreshadow the countless vicissitudes of human life. Hence the phenomena of the planetary movements were watched with feelings of superstitious awe; and all the particulars relating to them were carefully recorded for future guidance. It is in Asia, the seat of all the early inventions of mankind, that we discern the dawn of this sublime science. The annals of the Chinese contain the earliest records of celestial phenomena; but the Chaldean observ ations are more interesting to Europeans on account of their connexion with modern astronomy. The serene skies and mild climate of central Asia were eminently favourable for contemplating the heavens. Accord ingly we find that at Babylon eclipses and other phenomena were assiduously observed from a very remote antiquity. But mere observation cannot constitute science. Facts, however carefully recorded, must be subsequently scrutinized, compared, and classified, before any general conclusions can be derived from them, relative to the arrangements of the material universe. The astronomers of Asia, although patient observers, do not appear to have in any age aspired to this more exalted occupation of the mind. The Greeks first reduced the knowledge relative to the celestial motions into a systematic form. This object was not, however, effected during the early period of Grecian history. The Chaldeans, by confining their attention to the mere occurrence of phenomena, were unable to arrive at general views of the celestial motions; the philosophers of the Grecian schools, on the other hand, long wasted their transcendent talents in groundless speculations, which were equally ineffectual in producing any permanent influence on the progress of Astronomy. Amid the numberless ideas which perpetually occurred to the speculative minds of the Greek philosophers, it is perhaps not surprising that the true system of the world should have suggested itself to them. Pythagoras is said to have taught his followers that the sun is placed immoveable in the centre of the universe; and that the earth moves round him in an annual orbit. This system was first taught publicly by Philolaus, and was adopted by several ancient philosophers. Nicetas of Syracuse, on the other hand, is said to have explained the diurnal appearance of the heavens by the motion of the earth round a fixed axis. The ultimate abandonment of these sublime doctrines by the Greek philosophers, has been attributed to the hostility of the Aristotelians, who had placed the earth immoveable in the centre of the universe. It is doubtful, however, whether they were at any time supported by sound arguments drawn from observation. We know at least that the Pythagoreans, like the other sects of the Greek philosophers, were more prone to indulge in speculation than to examine facts. It was not until the reign of the Ptolemies commenced at Alexandria, that Astronomy, under the munificent patronage of those princes, was cultivated as a science of observation and theory. HIPPARCHUS, who flourished about the year 160 A.C., is the most illustrious astronomer of antiquity. The island of Rhodes is known to have been the principal scene of his labours. He is also alleged to have made observations at Alexandria; but this is a point which cannot be easily decided. This great man was at once a mathematician, an observer, and a theorist; and in all these capacities he exhibited powers of genius of the highest order: only two or three individuals can rank with him in the history of physical science. We owe to him the earliest catalogue of the stars, and the first theories of the sun and moon, in which their motions were submitted to strict calculation. He also executed the greater portion of the observations for a similar theory of the planets; discovered the precession of the equinoxes, and invented the sciences of plane and spherical trigonometry. He represented the motions of the sun and moon by means of epicycles revolving on circular orbits. This ingenious hypothesis had been already imagined by the Greek philosophers; but it proved of little value so long as it was unaccompanied by a calculus. Hipparchus supplied this desideratum by his invention of trigonometry, and computed tables of the sun and moon. The epicyclical theory did not indeed accord with the real state of the heavens; but it served the valuable purpose of enabling the astronomer to group together the facts derived from observation, and to predict the places of the celestial bodies with all the accuracy demanded by the existing condition of practical science. Nor should it be forgotten, in estimating the merits of this theory, that it was by a comparison of its results with those derived from actual observation that the real nature of the planetary motions was finally discovered. The most eminent astronomer of ancient times after Hipparchus is PTOLEMY, who flourished about the year 140 A.D. He devoted his attention chiefly to the task of extending and improving the theories of Hipparchus. He established the theory of the planets in accordance with the principles of that astronomer. He also discovered the inequality in the moon's longitude, termed the evection, and was the first who pointed out the effect of refraction in altering the place of a heavenly body. He is the author of a treatise on Astronomy called in Greek the Syntaxis, but which has been more frequently designated by the Arabian name of the Almagest. This work, which has come down to us entire, is remarkable for containing nearly all the knowledge we possess of the ancient Astronomy. Ptolemy adopted as the basis of his work, the system of the world which places the earth immoveable in the centre of the universe, the sun, moon, and planets revolving severally in orbits of different magnitudes, and the whole heavens turning round it every twenty-four hours. This system has been termed the Ptolemaic, because it was defended by the author of the Syntaxis; but, if we are to look for its origin, we must ascend to a much higher antiquity. With the irruption of the followers of Mahomet into Egypt, and the destruction of the famous library of Alexandria, about the middle of the seventh century, the science of Astronomy, which had long been declining among the Greeks, finally ceased altogether to be cultivated by that people. The Arabians, who now succeeded to the empire of the civilized world, devoted themselves with laudable assiduity to the study of the Greek authors, and Bagdad henceforth assumed the place of Alexandria, as the centre of literature and philosophy. Astronomy was cultivated by them with great ardour; but, like all other oriental nations, they exhibited an incapacity for speculation, and consequently the science did not acquire any extension from their labours. They generally adhered with superstitious reverence to the theories of the Greek astronomers, which they sought to amend only by means of more accurate observations. In the practical department of the science they indeed displayed a marked superiority to their masters, whose natural genius was averse to the monotonous task of observation. The Arabian astronomers may be said to have acted merely as the faithful guardians of science until the progress of events transferred it to a race of greater intellectual vigour. After ages of profound slumber, Western Europe finally awoke to pursue her glorious career. In the ninth and tenth centuries, several enlightened persons travelled from France and England into Spain, to study mathematics and astronomy at the Moorish universities, and upon their return home diffused a knowledge of those sciences among their countrymen. In the thirteenth century the Almagest was translated from Arabic into Latin, under the auspices of the emperor Frederick the Second. This step was attended with the most beneficial consequences to the study of Astronomy, which was now rendered generally accessible to persons of learning throughout all those countries where the Latin language prevailed. In the thirteenth century, Alphonso X., King of Castile, conferred a great benefit on science by causing the publication of new tables of Astronomy. They were executed at an immense expense, under the superintendence of the most eminent astronomers who could be found at the Moorish universities. Alphonso is reported to have said of the prevailing system of Astronomy, teeming with "Cycle upon epicycle, orb on orb," that if the Deity had consulted him at the creation of the world he would have given him good advice. This remark, though irreverent in the highest degree, was doubtless meant to convey a censure upon the cumbrous mechanism by which the system of the world was represented, rather than upon the actual arrangements of the system itself. About the close of the fifteenth century the study of Astronomy received a great impulse from the labours of Purbach and Regiomontanus, two Germans of very original genius. They introduced some modification of the ancient theories, and improved the methods of calculation. Nearly about the same time the art of observation was revived by Waltherus, an astronomer of considerable merit, and a native also of Germany. NICHOLAS COPERNICUS, the restorer of the true system of Astronomy, was born at Thorn, a town in Polish Prussia, on the 12th of February, 1473*. This illustrious man was gifted with a profound sagacity, which enabled him to distinguish the genuine principles of nature from the contrivances of the human imagination. He had long meditated on the system of the world, and was struck with the complication of the theory representing it, when contrasted with the harmony which everywhere pervaded the arrangements of creation. The earth was placed immoveable in the centre of the universe, while the sun, moon, and planets, and even the starry heavens, revolved round it with inconceivable velocities. He, however, considered it impossible to reconcile this hypothesis with the variable appearance presented by the superior planets in different parts of their orbits relative to the sun. He remarked especially that when Mars was in opposition, he almost rivalled Jupiter in brilliancy, while towards conjunction he dwindled to a star of the second magnitude. This fact appeared to him to offer irresistibly conclusive evidence that the earth could not be the centre of the planet's motion. He now began to ponder upon the opinions of some ancient philosophers on this subject. He found in the writings of Martianus Capella an opinion ascribed to the Egyptians, which supposed Mercury and Venus to revolve in orbits round the sun, while they accompanied him in his annual motion round the earth. He perceived that this theory would offer a most satisfactory account of the alternate appearance of the planets on each side of the sun, and would also determine the limit of their digressions. The increasing magnitude of the Copernicus died in the year 1543. He was of Sclavonic extraction. His grandfather, Nicholas Copernicus, was a native of Bohemia; but about the close of the fourteenth century he removed to Poland, and established himself in Cracow. His name appears inscribed in the records of that city for the year 1396. His Bohemian origin was duly attested on the occasion of his enrolment. |