Until nearly a hundred years ago the stars were regarded by practical astronomers mainly as a number of convenient fixed points by which the motions of the various members of the solar system could be determined and compared. Their recognised function, in fact, was that of milestones on the great celestial highway traversed by the planets, as well as on the byeways of space occasionally pursued by comets. Not that curiosity as to their nature, and even conjecture as to their origin, were at any period absent. Both were from time to time powerfully stimulated by the appearance of startling novelties in a region described by philosophers as “incorruptible,” or exempt from change. The catalogue of Hipparchus probably, and certainly that of Tycho Brahe, some seventeen centuries later, owed each its origin to the temporary blaze of a new star. The general aspect of the skies was thus (however imperfectly) recorded from age to age, and with improved appliances the enumeration was rendered more and more accurate and complete; but the secrets of the stellar sphere remained inviolate.

In a qualified, though very real sense, Sir William Herschel may be called the Founder of Sidereal Astronomy. Before his time some curious facts had been noted, and some ingenious speculations hazarded, regarding the condition of the stars, but not even the rudiments of systematic knowledge had been acquired. The facts ascertained can be summed up in a very few sentences.

In our chapters on the Sun and Moon, on the Earth and Venus, and on Mercury and Mars, we have been usually discussing the features and the movements of globes of vast dimensions. The least of all these bodies is the moon, but even our moon is a ball 2,000 miles from one side to the other. In approaching the subject of the minor planets∧ we must be prepared to find planets of dimension? quite inconsiderable in comparison with the great globes of our system. No doubt these minor planets are all of them some few miles, and some of them a great many miles, in diameter. Were they close to the earth they would be conspicuous, and even splendid objects; but as they are so distant they do not, even in our greatest telescopes, become very remarkable, while to the unaided eye they are totally invisible.

In a diagram of the orbits of the various planets, it is shown that a wide space exists between the orbit of Mars and the orbit of Jupiter. It was often surmised that this wide region must be tenanted by some other planet. The presumption became much stronger when a remarkable law was discovered which exhibited, with considerable accuracy, the relative distances of the great planets of our system.

The zeal with which solar studies have been pursued during the last quarter of a century has already gone far to redeem the neglect of the two preceding ones. Since Schwabe's discovery was published in 1851, observers have multiplied, new facts have been rapidly accumulated, and the previous comparative quiescence of thought on the great subject of the constitution of the sun, has been replaced by a bewildering variety of speculations, conjectures, and more or less justifiable inferences. It is satisfactory to find this novel impulse not only shared, but to a large extent guided, by our countrymen.

William Rutter Dawes, one of many clergymen eminent in astronomy, observed, in 1852, with the help of a solar eyepiece of his own devising, some curious details of spot structure. The umbra–heretofore taken for the darkest part of the spot–was seen to be suffused with a mottled, nebulous illumination, in marked contrast with the striated appearance of the penumbra; while through this “cloudy stratum” a “black opening” permitted the eye to divine further unfathomable depths beyond. The hole thus disclosed–evidently the true nucleus–was found to be present in all considerable, as well as in many small maculæ.

Again, the whirling motions of some of these objects were noticed by him. The remarkable form of one sketched at Wateringbury, in Kent, January 17, 1852, gave him the means of detecting and measuring a rotatory movement of the whole spot round the black nucleus at the rate of 100 degrees in six days.

In the course of his early gropings towards a law of the planetary distances, Kepler tried the experiment of setting a planet, invisible by reason of its smallness, to revolve in the vast region of (seemingly) desert space separating Mars from Jupiter. The disproportionate magnitude of the same interval was explained by Kant as due to the overweening size of Jupiter. The zone in which each planet moved was, according to the philosopher of Königsberg, to be regarded as the empty storehouse from which its materials had been derived. A definite relation should thus exist between the planetary masses and the planetary intervals. Lambert, on the other hand, sportively suggested that the body or bodies (for it is noticeable that he speaks of them in the plural) which once bridged this portentous gap in the solar system, might, in some remote age, have been swept away by a great comet, and forced to attend its wanderings through space.

These speculations were destined before long to assume a more definite form. Johann Daniel Titius, a professor at Wittenberg (where he died in 1796), pointed out in 1772, in a note to a translation of Bonnet's Contemplation de la Nature, the existence of a remarkable symmetry in the disposition of the bodies constituting the solar system. By a certain series of numbers, increasing in regular progression, he showed that the distances of the six known planets from the sun might be represented with a close approach to accuracy.

The mystery of comets' tails has been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. We have, then, the assurance that these extraordinary appendages are composed of no ethereal or super-sensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity. This is unquestionably one of the most remarkable discoveries of our time.

Olbers, as already stated, originated in 1812 the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. It was developed and enforced by Bessel's discussion of the appearances presented by Halley's comet in 1835. He, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. Professor W. A. Norton of Yale College considerably improved this by inquiries begun in 1844, and resumed on the apparition of Donati's comet; and Dr. C. F. Pape at Altonax gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object.

Newton was the first who attempted to measure the quantity of heat received by the earth from the sun. His object in making the experiment was to ascertain the temperature encountered by the comet of 1680 at its passage through perihelion. He found it, by multiplying the observed heating effects of direct sunshine according to the familiar rule of the “inverse squares of the distances,” to be about 2000 times that of red-hot iron.

Determinations of the sun's thermal power made with some scientific exactness, date, however, from 1837. A few days previous to the beginning of that year, Herschel began observing at the Cape of Good Hope with an “actinometer,” and obtained results agreeing quite satisfactorily with those derived by Pouillet from experiments made in France some months later with a “pyrheliometer.” Pouillet found that the vertical rays of the sun falling on each square centimetre of the earth's surface are competent (apart from atmospheric absorption) to raise the temperature of 1.7633 grammes of water one degree centigrade per minute. This number (1.7633) he called the “solar constant;” and the unit of heat chosen is known as the “calorie.” Hence it was computed that the total amount of solar heat received during a year would suffice to melt a layer of ice covering the entire earth to a depth of 30.89 metres, or 100 feet; while the heat emitted would melt, at the sun's surface, a stratum 11.80 metres thick each minute.

In the preceding chapters we have dealt with the gigantic bodies which form the chief objects in what we know as the solar system. We have studied mighty planets measuring thousands of miles in diameter, and we have followed the movements of comets, whose dimensions are to be told by millions of miles. Once, indeed, in a previous chapter, we have made a descent to objects much lower in the scale of magnitude, and we have examined that numerous class of small bodies which we call the minor planets. It is now, however, our duty to make a still further, and this time a very long step, downwards in the scale of magnitude. Even the minor planets must be regarded as colossal objects, when compared with those little bodies whose presence is revealed to us in a most interesting, and sometimes in a most striking manner.

These small bodies compensate in some degree for their minute size, by the enormous profusion in which they exist. No attempt, indeed, could be made to tell in figures the myriads in which they swarm throughout space. They are probably of very varied dimensions, some of them∧ being many pounds or perhaps tons in weight, while others seem to be not larger than pebbles, or even than grains of sand. Yet, insignificant as these bodies may seem, the great sun himself does not disdain to accept their control. Each particle, whether it be as small as the mote in a sunbeam or as mighty as the planet Jupiter, will perform its path around the sun in conformity with the laws of Kepler.

Comparing the methods now available for astronomical inquiries with those in use thirty years ago, we are at once struck with the fact that they have multiplied. The telescope has been supplemented by the spectroscope and the photographic camera. Now this really involves a whole world of change. It means that astronomy has left the place where she dwelt apart in rapt union with mathematics, indifferent to all things on earth save only to those mechanical improvements which should aid her to penetrate further into the heavens, and has descended into the forum of human knowledge, at once a suppliant and a patron, alternately invoking help from, and promising it to each of the sciences, and patiently waiting upon the advance of all. The science of the heavenly bodies has, in a word, become a branch of terrestrial physics, or rather a higher kind of integration of all their results. It has, however, this leading peculiarity, that the materials for the whole of its inquiries are telescopically furnished. They are such as the unarmed eye takes no, or a very imperfect cognisance of.

Spectroscopic and photographic apparatus are simply additions to the telescope. They do not supersede, or render it of less importance. On the contrary, the efficacy of their action depends primarily upon the optical qualities of the instrument they are attached to. Hence the development, to their fullest extent, of the powers of the telescope is of vital moment to the progress of modern physical astronomy, while the older mathematical astronomy could afford to remain comparatively indifferent to it.

We have now to consider labours of a totally different character from those of Sir William Herschel. Exploration and discovery do not constitute the whole business of astronomy; the less adventurous, though not less arduous, task of gaining a more and more complete mastery over the problems immemorially presented to her, may, on the contrary, be said to form her primary duty. A knowledge of the movements of the heavenly bodies has, from the earliest times, been demanded by the urgent needs of mankind; and science finds prosperity, as in many cases it has taken its origin, in condescension to practical claims. Indeed, to bring such knowledge as near as possible to absolute precision has been defined by no mean authority as the true end of astronomy.

Several causes concurred about the beginning of the present century to give a fresh and powerful impulse to investigations having this end in view. The rapid progress of theory almost compelled a corresponding advance in observation; instrumental improvements rendered such an advance possible; Herschel's discoveries quickened public interest in celestial inquiries; royal, imperial, and grand-ducal patronage widened the scope of individual effort. The heart of the new movement was in Germany. Hitherto the observatory of Flamsteed and Bradley had been the acknowledged centre of practical astronomy; Greenwich observations were the standard of reference all over Europe; and the art of observing prospered in direct proportion to the fidelity with which Greenwich methods were imitated.

The progress of astronomy during the last hundred years has been rapid and extraordinary. In its distinctive features, moreover, the nature of that progress has been such as to lend itself with facility to untechnical treatment. To this circumstance the present volume owes its origin. It embodies an attempt to enable the ordinary reader to follow, with intelligent interest, the course of modern astronomical inquiries, and to realise (so far as it can at present be realised) the full effect of the comprehensive change in the whole aspect, purposes, and methods of celestial science introduced by the momentous discovery of spectrum analysis.

Since Professor Grant's invaluable work on the History of Physical Astronomy was published, a third of a century has elapsed. During the interval, a so-called “new astronomy” has grown up by the side of the old. One effect of its advent has been to render the science of the heavenly bodies more popular, both in its needs and in its nature, than formerly. More popular in its needs, since its progress now primarily depends upon the interest in, and consequent efforts towards its advancement of the general public; more popular in its nature, because the kind of knowledge it now chiefly tends to accumulate is more easily intelligible–less remote from ordinary experience–than that evolved by the aid of the calculus from materials collected by the use of the transit-instrument and chronograph.