Astronomical observation

“ Whatever advances may be made in astronomical science, theoretical or applied, I am strongly inclined to think that they will be made in connection with an increased command of instrumental power. The natural order in which the human mind proceeds in the acquisition of astronomical knowledge is minute and accurate observation of the phenomena of the heavens, the skillful discussion and analysis of these observations, and sound philosophy in generalizing the results. ”

“ During the latter half of the nineteenth century discoveries were made which extended the powers of astronomical research into entirely new and unexpected regions, comparable to those which were opened up by the discovery of the telescope more than two centuries before. The older astronomy for more than two thousand years was purely mechanical and mathematical, being limited to observation and measurement of the apparent motions of the heavenly bodies, and the attempts to deduce, from these apparent motions, their real motions, and thus determine the actual structure of the solar system. ”

“ Astronomical observations and experiments selected for the purpose of ascertaining the relative distances of clusters of stars, and of investigating how far the power of our telescopes may be expected to reach into space, when directed to ambiguous celestial objects. ”

“ The science is concerned with the heavenly bodies. The earth on which we live is, to all intents and purposes, one of these bodies, and, so far as its relations to the heavens are concerned, must be included in astronomy. The processes of measuring great portions of the earth, and of determining geographical positions, require both astronomical observations proper, and determinations made with instruments similar to those of astronomy. Hence geodesy may be regarded as a branch of practical astronomy. (S. N.)
History of Astronomy. ”

“ This eminent astronomer finds that stars with measurable proper motions—and therefore in all probability relatively near the earth—have mostly spectra of the solar type, and seem to cluster round “a point adjacent to the sun, in total disregard to the position of the Milky Way,” and that stars with little or no proper motion collect round the galactic plain. ”

“ Astronomers who make a specialty of solar physics have, especially in this country, so many other duties, and their view is so often interrupted by clouds, that a continuous record of the spots on the sun and the changes they undergo is hardly possible. Perhaps one of the most interesting and useful pieces of astronomical work which an amateur can perform will consist of a record of the origin and changes of form of the solar spots and faculae. ”

“ If we consider with attention the periodical variations of the spots we shall not be long in coming to the conclusion that it is impossible to connect them directly with any one astronomical function in particular, for the spots appear in a sudden and irregular manner which contrasts in a striking degree with the continuous and progressive action of the ordinary perturbations which we meet with in the study of Celestial Mechanics. ”

“ In this favored branch of knowledge, the most recondite and delicate discoveries can no more suffer doubt or contradiction, than the most palpable facts of sense which the face of nature offers to our notice. The last great discovery in astronomy—the motion of the stars arising from Aberration—is as obvious to the vast population of astronomical observers in all parts of the world, as the motion of the stars about the pole is to the casual night wanderer. ”

“ In the case of the motion of the moon round the earth, or of the earth round the sun, cases in which variations in distance could not readily be observed, epicycles might therefore be expected to give a satisfactory result, at any rate until methods of observation were sufficiently improved to measure with some accuracy the apparent sizes of the sun and moon, and so check the variations in their distances. ”

“ Successive astronomers in different ages and countries have observed the celestial orbs, and swept the heavens, and for ever bring us back the same story of the number, the dimensions, the distances, and the arrangement of the heavenly bodies which form the subject of astronomical science. ”

“ Those wonderful varieties in the appearance of the solar surface which have so long perplexed astronomers, not only supply in themselves interesting subjects of observation and examination, but gain an enhanced meaning from the consideration that they speak meaningly to us of the structure of an orb which is the source of light and heat enjoyed by a series of dependent worlds whereof our earth is—in size at least—a comparatively insignificant member. ”

“ His first important astronomical publication contained an investigation of the changes which the orbits of several of the planets, including the earth, have undergone in times past, and which they will undergo in times to come.
As an illustration of these researches, we may take the case of the planet in which we are, of course, especially interested, namely, the earth, and we can investigate the changes which, in the lapse of time, the earth's orbit has undergone, in consequence of the disturbance to which it has been subjected by the other planets. ”

“ As, on the one hand, the complete calculation of the perturbations of the various bodies of the solar system presupposes a knowledge of their masses, so reciprocally if the magnitudes of these disturbances can be obtained from observation they can be used to determine or to correct the values of the several masses. ”

“ Perhaps the differences between the state of our knowledge of the astronomical and of the meteorological phenomena will be more conveniently explained by choosing a branch of astronomical science with which we are at present only imperfectly acquainted. ”

“ The other system of astronomical speculation takes its start from the sun as the center and the governor of the planetary system, and finds that sun himself, with all his dependent orbs, marching onward through the heavens. And we know what comes of that. (There came a book from Dr. Hill, years ago, which I read with the intensest interest, concerning the relation of the stars.) [Applause.] There comes order and harmony in all the system of the heavens. We measure and weigh the planet in its course. ”

“ Observations of occultations of stars by the Moon or the planets lead to the same result. Theoretical astronomy tells us the distance between the planets and the Sun; physics tell us the quantity of light received by each planet from the Sun; and the period of revolution and the inclination of the planet’s axis tell us about the course of its seasons. ”

“ The geocentric system of astronomy, with its eccentrics and its epicycles, was an hypothesis utterly at variance with fact, which nevertheless did great things for the advancement of astronomical knowledge. ”

“ So the continuance for more than two centuries of daily observations of places of moon, stars, and planets is likewise 'monotonous, wearing, and almost inglorious;' the one compensation is that it is essential to the life of astronomy. ”

“ The solar spectrum will perhaps one day, by its recurrent modifications, tell us something of impending droughts, famines, and cyclones.
Astronomy generalises the results of the other sciences. She exhibits the laws of Nature working over a wider area, and under more varied conditions, than ordinary experience presents. ”

“ Astronomy proper deals with the position of the earth in space and its relation to the other heavenly bodies, whether suns, fixed stars, planets, satellites, comets, or other bodies in the vast space about us. Meteorology deals with the atmosphere of the globe, in all its forms. ”

“ They would still appear to an observer in the solar system to have motions, and part of these motions would be truly proper to the stars, and part would be due to the advance of the sun itself in space.
Observations can show us only the resultant of these two motions. It is for reasoning to separate this resultant into its two components. At first the question is to determine [Pg 152] whether the results of observation indicate any solar motion at all. If there is none, the proper motions of stars will be directed along all possible lines. ”

“ How much there may be beyond what our telescopes show, we cannot tell ; but what we can know is of unimaginable immensity. In the visible world, the milky way is a tiny fragment; within this frag- ment, the solar system is an infinitesimal speck, and of this speck our planet is a microscopic dot. ”

“ But astronomers have constantly to deal with more complicated cases in which two or more unknown quantities have to be determined from observations of different quantities, as, for example, when the elements of the orbit of a planet (chapter xi., § 236) have to be found from observations of the planet's position at different times. ”

“ Again, men wished to learn the true shape of the earth, and went hither and thither measuring its globe; but the moon, meanwhile, told the astronomer who remained at home a truer tale. They sought to learn the earth’s distance from the sun, and from this and that point they turned their telescopes on Venus in transit; but the moon set them nearer the truth, and that not by a few miles, but by 2,000,000 miles or more. We shall see that she has had something to say about our great terrestrial time-piece.
One of the great charms of the science of astronomy is, that it enables men to predict. ”

“ It is even probable that space contains many darkened orbs, stars that may have risen in constellations to adorn the skies of prehistoric time—now cold, unseen, unknown. So far from dealing with an unvarying universe, it is safe to say that sidereal astronomy can advance only by the discovery of change. Observational science watches with untiring industry, and night hides few celestial events from the ardent scrutiny of astronomers. ”

“ With regard to the moon, sun, and some of the planets, in addition to their form and size, we have been able, by this means, to obtain some slight knowledge of their physical structure. But, with reference to the myriads of stars, clusters, and nebulæ, which people the depths of space, the telescope reveals little more than variety in color, brightness, and shape. ”

“ But the progress of investigation has changed all this, and has brought physical and natural science back to a position nearer to that of the old cosmogonies. Physical astronomy has shown that the constant emission of heat and light from the sun and other stars must have had a beginning, and is hurrying on toward an end, that the earth and its satellite the moon are receding from each other, and that even the spinning of our globe on its - 5 - axis is diminishing in rapidity. ”

“ Spectroscopic investigations of stellar movements may confidently be expected to play a leading part in the unravelment of the vast and complex relations which we can dimly detect as prevailing among the innumerable orbs of the sidereal world; for it supplements the means which we possess of measuring by direct observation movements transverse to the line of sight, and thus completes our knowledge of the courses and velocities of stars at ascertained distances, while supplying for all a valuable index to the amount of perspective foreshortening of apparent movement. ”

“ What terms too grand to describe the lofty speculations of the astronomer, who points his telescope to some dark point in the blue sky, and descries in its infinite depths a cluster of closely aggregated shining particles, minute as the motes in the sunbeam, and hails it as the discovery of a new system of worlds. He cannot count them, for they are innumerable. ”

“ The obvious fact that the sun, the moon, and the stars rose day by day, moved across the sky in a glorious never-ending procession, and duly set when their appointed courses had been run, demanded some explanation. The circumstance that the fixed stars preserved their mutual distances from year to year, and from age to age, appeared to Ptolemy to prove that the sphere which contained those stars, and on whose surface they were believed by him to be fixed, revolved completely around the earth once every day. ”

“ Henceforth astronomy possesses a new instrument of discovery, and also a new tribunal to which all speculations about the sun and the stars, the aurora and the zodiacal light, the meteors and the comets, must be brought and by which they must be judged. ”

“ GRAVITATIONAL ASTRONOMY IN THE 18TH CENTURY.
"Astronomy, considered in the most general way, is a great problem of mechanics, the arbitrary data of which are the elements of the celestial movements; its solution depends both on the accuracy of observations and on the perfection of analysis."
Laplace, Preface to the Mécanique Céleste. 228. The solar system, as it was known at the beginning of the 18th century, contained 18 recognised members: the sun, six planets, ten satellites (one belonging to the earth, four to Jupiter, and five to Saturn) , and Saturn's ring. ”

“ Moreover modern astronomers, as well as ancient, find it convenient for very many purposes to make use of this sphere, though it has no material existence, as a means of representing the directions in which the heavenly bodies are seen and their motions. ”

“ Perhaps most astronomers now believe, on what they think sufficient evidence, that the celestial spaces are occupied by innumerable small bodies moving round the sun, of whose nature and orbits nothing is certainly known. ”