In the early part of his career, John Herschel held firmly to the views of his father of the difference between star-clusters and nebulæ, considering the latter to be composed of “shining fluid.” But he fell off from this view with the resolution into stars of many irresolvable nebulæ. In 1845 William Parsons, third Earl of Rosse (1800-1867), erected at Birr Castle, in Ireland, his great 6-foot reflector, which still surpasses all other telescopes in point of size. With this instrument Lord Rosse believed himself to have resolved the Crab nebula in Taurus and the Nebula in Orion, which was also said to have been resolved by Bond with the 15-inch refractor at Harvard; and in 1854 Olmsted declared the “resolution” of these nebulæ to be the signal for the renunciation of Herschel’s nebular theory. Most astronomers fell in with the view that all the nebulæ were distant clusters, which would eventually be resolved into stars, although it is only right to state that the Scottish astronomer, John Pringle Nichol (1804-1859), and some other investigators, held to the theory of Herschel.
The solution of the great problem was in 1864, when on August 29 of that year Huggins turned his spectroscope on a bright planetary nebula in Draco. To his amazement the spectrum was one of bright lines, proving conclusively that the nebula was not a star-cluster, but a mass of glowing gas,—hydrogen, and some other unknown substance, now named “nebulium.” By 1868 Huggins had observed the spectra of seventy nebulæ. Of these one-third proved to be gaseous, among them the great Orion nebula which Lord Rosse was believed to have resolved into stars. In the spectrum of the latter, the “chief nebular line” was at first ascribed by Huggins to nitrogen, but this was a mistake. Later, it was believed by Lockyer to coincide with the fluting of magnesium, but this was disproved by Huggins in 1889-90, and by Keeler in 1890-91. The great nebula in Andromeda and the great spiral in Canes Venatici were found by Huggins to display a continuous spectrum, and a similar discovery was made in regard to the cluster M 13 in Hercules, and other star-clusters. In the case of the nebulæ, it is not believed that the continuous spectrum is due to the existence of sun-like bodies, as a gas under pressure would give a continuous spectrum.
The Orion nebula has been more thoroughly studied than any other object of its class. The application of photography to spectroscopy has done much to further the study of the lines in the nebular spectrum. In 1886 Copeland detected in the spectrum of the Orion nebula the yellow ray of helium. On February 13, 1890, Scheiner announced an important discovery, namely, the possession by both the nebula and the stars in Orion—with the exception of Betelgeux—of a line, which appeared bright in the nebular spectra and dark in the stellar. This line was identified by Vogel, Lockyer, and others with that of helium.
Nebular photography was inaugurated in 1880 by Draper, who obtained a remarkably good representation of the Orion nebula in that year. His work in this direction, cut short by his death in 1882, was taken up by Janssen at Meudon, and by Common in England, who obtained, in 1883, several excellent photographs. Later photographs have shown the Orion nebula to be much more extended than visual observations would lead one to expect. A photograph secured in 1890 by W. H. Pickering revealed the nebulous matter in Orion in its true form, that of a gigantic spiral, starting from near Bellatrix, sweeping past κ Orionis and Rigel to η, and joining with the great nebula surrounding θ; the entire constellation being thus shown to be enwrapped in nebulous haze.
In 1885 nebular photography was commenced by Isaac Roberts (1829-1904), the English amateur astronomer, who secured admirable representations of clusters and nebulæ. He published, in 1893 and 1900, two volumes of collected photographs of clusters and nebulæ. This monumental work was thus referred to by Dr William James Lockyer: “Dr Roberts has not only nobly enriched astronomical science, but has raised a monument to himself which will last as long as astronomy has any interest for mankind.”
Perhaps the most remarkable revelation made by photography in this branch of research has been the discovery of the nebulæ in the Pleiades. In 1859 Tempel observed at Florence an elliptical nebula south of the star Merope. On November 16, 1885, the brothers Henry obtained at Paris a photograph of the Pleiades, revealing the existence of a small spiral nebula. This was confirmed by visual observations, and particularly by the photographs of Roberts, which also showed the entire cluster to be nebulous, and that “the nebulosity extends in streamers and fleecy masses, till it seems almost to fill the spaces between the stars, and to extend far beyond them.” In 1888 a further advance was made by the brothers Henry, who found seven stars to be strung on a nebulous streak.
Since 1890 nebular photography has been pursued by Max Wolf in Germany, and by E. E. Barnard and J. E. Keeler in America. Wolf’s photographs of the constellation Cygnus brought out the close connection between the stars and the extensively diffused nebulosities discovered by him. In 1901 Wolf discovered a “nebelhaufen” or cluster of nebulæ, and in 1902 published a catalogue of 1528 nebulæ round the pole of the Galaxy, showing them to be systematically distributed. Keeler made his memorable observations with the great 36-inch reflecting telescope, which was constructed in England many years ago by Common. It afterwards passed into the hands of Mr Crossley of Halifax, who presented it to the Lick Observatory. With this great instrument Keeler commenced to take photographs of the heavens. On one occasion he photographed a well-known nebula, and on developing the plate was surprised to find seven new nebulæ besides that which he had photographed. On another occasion he exposed a plate to a nebula in Pegasus. He was amazed to find altogether twenty-one nebulæ included in the photograph. To give another instance, a plate directed to the constellation Andromeda contained no fewer than thirty-two nebulous objects. This has given an enormous extension to our knowledge of the nebulæ. But even this is not all. Keeler found on his plates numerous points of light which seem to be also nebulæ, either too small or too remote to appear as such. Apparently, however, they are not stars. Keeler’s work convinced him that, on a modest estimate, there must be at least one hundred and twenty thousand new nebulæ within reach of the Crossley reflector. Half of these, he announced, were probably spiral. An idea of the vast importance of Keeler’s work may be gained if we reflect that the observations of all the earlier astronomers resulted in the discovery of six thousand nebulæ. The investigations of Keeler, in all probability, were the means of adding 120,000 more.
Many observations have been made on nebulæ, for the purpose of ascertaining their proper motions—but without success. Measurements were made by D’Arrest in 1857 and by Burnham in 1891, but none of these revealed any motion of the nebulæ across the line of sight. Even the new spectroscopic method of determining motions in the line of sight, in the hands of Huggins, failed in the case of the nebulæ. With the great Lick refractor at his disposal, Keeler attacked the subject in 1890, and measured the radial velocities of ten nebulæ. He found that the well-known planetary nebula in Draco was moving towards the Solar System at the rate of 40 miles a second; for the Orion nebula he found a motion of recession of 11 miles a second; but probably this belongs chiefly to the movement of the Solar System in the opposite direction.
Unfortunately Keeler did not live to carry on his investigations in nebular astronomy. His early death brought to an abrupt end these fruitful investigations. Appointed director of the Lick Observatory in 1898, he died suddenly at San Francisco on August 12, 1900, at the early age of forty-two.
CHAPTER XII.
STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE.
After the death of Herschel there was little done in the direction of furthering our knowledge of stellar distribution, or the construction of the heavens. Here, as elsewhere, Herschel’s immediate successor was his son, whose star-gauges, both in England and in South Africa, were a worthy sequel to those of his father; but John Herschel, in his books on astronomy, reproduced his father’s disc-theory, unaware that the elder Herschel had himself abandoned it. The work of the younger Herschel was entirely supplementary to that of his father.
To Wilhelm Struve belongs the credit of showing the disc-theory to be untenable, and of demonstrating that Herschel had abandoned it. This he was able to do after a perusal of Herschel’s papers, presented to him by John Herschel. Having demonstrated this, he undertook a series of investigations which resulted in his famous theory of the Universe. This was published in his work ‘Études d’Astronomie Stellaire,’ which was published in 1847. His researches were based on the star-catalogues of Bessel, Piazzi, and others; and dealing with 52,199 stars, he discussed the number of stars in each zone of Right Ascension. He found, in the words of Mr Gore, “that the numbers increase from hour i to hour vi, where they attain a maximum. They then diminish to a minimum at hour xiii, and rise to another but smaller maximum at hour xviii, again decreasing to a second minimum at hour xxii. As the hours vi and xviii are those crossed by the Milky Way, the result is very significant.” He concluded the Galaxy to be produced by a collection of irregularly-condensed clusters, the stars condensed in parallel planes. Next, he considered the Universe as perhaps infinitely extended in the direction of the Galaxy, and accordingly he put forward the idea that the light from the fainter and more distant stars was extinguished in its passage through the ether of space, which he regarded as imperfectly transparent. The theory, as Struve propounded it, was disposed of by Sir John Herschel, who remarked that we were not permitted to believe that at one part of the sky our view was limited by extinction, while at another a clear view right through the Galaxy could be had; and by Robert Grant (1814-1892), director of the Glasgow Observatory, who showed that, were the theory true, the Galaxy should present a uniform appearance throughout its course. On the whole, Struve’s theory was no improvement on Herschel’s; for, as Encke pointed out, Struve’s theory was built on five assumptions, all of which were questionable.
At the time of Struve’s investigation Mädler, at Dorpat, was engaged in an attempt to solve the question of the construction of the heavens by quite another method, that of stellar proper motion. He determined to investigate the subject of proper motion in order to discover the central body of the Milky Way. If such a centre existed, however, the motions near it would be somewhat different from those in the Solar System. In our Solar System the planets nearest the Sun move swiftest, owing to the strength of the force of gravitation. In the Sidereal System, on the other hand, the movements at the centre, as Mädler pointed out, would be slowest. As there would be no very large preponderating body, the mutual attractions of the different stars would cause the bodies at the boundaries of the Universe to move faster than those at the centre, the central sun—the object of Mädler’s search—being in a state of rest relative to the Sidereal System. Mädler accordingly began to search the heavens for a region of sluggish proper motions.
In the constellation Taurus, Mädler noticed that the proper motions of the stars were very slow. The idea occurred to him that the bright red star Aldebaran might be the central sun, but its very large proper motion was obviously against this inference. Star after star was now subjected by Mädler to the most careful scrutiny. At length, after a laborious investigation, he announced that the star which fulfilled the conditions of a central body was Alcyone, the brightest of the Pleiades, a group possessed of no proper motion except that due to the sun’s drift in the opposite direction. In 1846 Mädler published his hypothesis in his elaborate work, ‘The Central Sun.’ He announced that his observations had led him to the conclusion that Alcyone occupied the centre of gravity of the Sidereal System, and was the point round which the stars of the Galaxy were all revolving. His profound imagination, however, did not stop here. This speculation led him to the sublime thought that the centre of the Universe was the Abode of the Creator. In 1847 Struve rejected Mädler’s theory as “much too hazardous,” and this has been the general opinion of astronomers. Mädler’s theory is now regarded as quite untenable.
Herschel’s earlier idea that the nebulæ were external galaxies was long held by the majority of astronomers, in preference to his later and more advanced ideas. The supposed resolution of the nebulæ by Lord Rosse’s telescope gave support to this external galaxy theory. It was clearly shown, however, by William Whewell (1794-1866) in 1853, and by Herbert Spencer (1820-1903) in 1858, that the systematic distribution of the nebulæ in regard to the stars precluded the possibility of their being external galaxies. This was confirmed by the spectroscopic discovery of the gaseous nature of some of the nebulæ, and by the later researches of R. A. Proctor. Not only did Proctor make fresh discoveries, but it fell to him to clear away the erroneous ideas regarding the construction of the heavens, and to put the study on a new basis. In 1870 Proctor plotted on a single chart all the stars, to the number of 324,198, contained in Argelander’s ‘Durchmusterung’ charts. This work gave the death-blow to the “disc-theory.” In his own words, “In the very regions where the Herschelian gauges showed the minutest telescopic stars to be most crowded, my chart of 324,198 stars shows the stars of the higher orders (down to the eleventh magnitude) to be so crowded, that by their mere aggregation within the mass they show the Milky Way with all its streams and clusterings. It is utterly impossible that excessively remote stars could seem to be clustered exactly where relatively near stars were richly spread.”
Proctor showed also that in all probability the stars composing the nebulous light of the Galaxy are much smaller than the brighter stars, and not at such a great distance as their faintness would lead us to suppose,—a conclusion confirmed by the work of Celoria. Proctor was not so fortunate in theorising as in direct investigation. He thought that the Magellanic clouds were probably external galaxies; and further, he put forward the idea that the Milky Way is a spiral, the gaps and coal-sacks being due to loops in the stream, but neither of these ideas has found favour with astronomers. But the chief work accomplished by Proctor was a revision of our knowledge of the Universe, which he thus describes: “Within one and the same region coexist stars of many orders of real magnitude, the greatest being thousands of times larger than the least. All the nebulæ hitherto discovered, whether gaseous and stellar, irregular, planetary, ring-formed, or elliptic, exist within the limits of the Sidereal System.”
Proctor’s discovery of the excess of bright stars on the Galaxy was confirmed by Jean Charles Houzeau (1820-1888), director of the Brussels Observatory. Some time later J. E. Gore carefully examined the positions of all the brighter stars in the northern and southern hemisphere. Following this, he made an enumeration of the stars in the atlas of Heis and in the charts constructed by Harding; the outcome of the investigation being to show that stars of each individual magnitude taken separately tend to aggregate on the Galaxy, the aggregation being noticed even in first-magnitude stars. Gore further pointed out many cases of close connection between the lucid stars and the galactic light. A similar investigation was undertaken by Schiaparelli in 1889. Schiaparelli, basing his work on the catalogue of Gould and the photometric measures of Pickering, constructed a series of planispheres which demonstrated the crowding of the lucid stars towards the plane of the Galaxy. These investigations were still further continued by Simon Newcomb, who demonstrated that “the darker regions of the Galaxy are only slightly richer in stars visible to the naked eye than other parts of the heavens, while the bright areas are between 60 and 100 per cent richer than the dark areas.” The Dutch astronomer, Charles Easton, finds a connection between the distribution of ninth-magnitude stars and the luminous and obscure spots in the Galaxy.
It was noticed by Gould, from observations made at Cordova, that “a belt or stream of bright stars appears to girdle the heavens very nearly in a great circle which intersects the Milky Way.” According to Gould, the belt includes Orion, Canis Major, Argo, Crux, Centaurus, Lupus, and Scorpio in the southern hemisphere, and Taurus, Perseus, Cassiopeia, Cepheus, Cygnus, and Lyra in the northern. This was interpreted by Celoria as indicating the existence of two galactic rings, but Gould considered the zone of bright stars to form with the Sun a subordinate cluster of about five hundred stars within the Galaxy.
Perhaps the most elaborate investigations on the structure of the Universe have been those of Kapteyn, commenced in 1891. In that year he demonstrated that stars are bluer and more easily photographed in the Galaxy than elsewhere, a discovery independently made by Gill at the Cape, and Pickering at Harvard. In 1893 Kapteyn announced his conclusions, derived from a novel method of studying the distance of the stars from their proper motions. In order to reach a definite idea of the distances of the stars, he made use of the component of the proper motion, measured at right angles to a great circle of the sphere which passes through a given star and the apex of the solar motion. He found that stars of the first spectral type have smaller proper motions than those of the second, indicating that stars of the second type are on the average nearer to the Solar System than those of the first, the near vicinity containing almost exclusively second-type stars. Kapteyn concluded that the group of second-type stars formed one system, named the solar cluster, which he considered to be roughly spherical in shape. In 1902 he abandoned this idea, retaining, however, his opinions as to the relative distances of the different types. That the second-type stars are nearer to the Sun than the first is, he remarked in a letter to the writer, incontrovertible.
In the investigation of the motions in, and extent of, the Universe, the name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace, in Nova Scotia, he went to the States in 1853. In 1862 he received an appointment at Washington Observatory, and he retained an official position until 1897. Throughout his scientific career he has been specially attracted by the question of the construction of the heavens, which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s investigations have shown that some of the stars are not permanent members of the Sidereal System, among them the swiftly-moving 1830 Groombridge. He has shown that the Stellar Universe does not possess that form of stability which is seen in the Solar System. Newcomb considers the Universe to be limited in extent, as opposed to the opinions of Struve and others, who believed it to be infinite. He has brought clearly before his readers a calculation, based on the known law that there are three times as many stars of any given magnitude as of that immediately brighter, the increase of number compensating for the decrease of brilliance. Were the Universe infinitely extended, the whole heavens would shine with the brilliance of the Sun. Newcomb, therefore, concludes that “that collection of stars which we call the Universe is limited in extent.”
Positive evidence that this is the case was obtained by Giovanni Celoria, now director of the Milan Observatory, in the course of a series of star-gauges at the north galactic pole. Using a small refractor, showing stars barely to the eleventh magnitude, he found he could see exactly the same number of stars as Herschel’s large reflector, indicating that increase of optical power will not increase the number of stars visible in that direction. Celoria’s observation can only be explained on the assumption that the Universe is limited in extent, as otherwise Herschel’s telescope should have shown more stars than Celoria’s, even granting an extinction of light,—a theory which Newcomb, Schiaparelli, and others have shown to be quite untenable. That the Universe is limited in extent is about all that is known for certain, although even this has been called in question, notably by E. W. Maunder and H. H. Turner. The problem of the construction of the heavens is by no means solved, although several more or less probable theories have been advanced.
A series of investigations on stellar distribution, from 1884 to 1898, led Hugo Seeliger, director of the Munich Observatory, to some remarkable deductions. He believes the Universe to be flattened at the galactic poles. The Galaxy is the zone of stellar condensation, and he concludes the distance of the Solar System from the inner border of the zone to be 500 times the distance of Sirius, while the external border is 1100 times that distance. The Universe is finite in extent, its limits being about 9000 light years from the Solar System. In Seeliger’s opinion the extinction of light may come into play beyond our Universe, and prevent us seeing other collections of stars.
The question of external universes is purely a hypothetical one, although there is undoubtedly much to be said in its favour. These universes have never been seen, and we can only speculate as to their existence. The last word on the subject is by Gore, in 1893, in his elaborate work, ‘The Visible Universe.’ He regards the Solar System as a system of the first order, and the Galaxy and its fellow-universes of the second. He makes a calculation of the possible distance of an external universe of his second order. He assumes the distance of the nearest universe from our Galaxy as proportional to that separating the Sun from α Centauri, and reaches the amazing conclusion that the distance of the nearest Galaxy is no less than 520,149,600,000,000,000,000 miles,—a distance which light, with its inconceivable velocity of 186,000 miles a second, would take almost ninety millions of years to traverse.
These calculations absolutely overwhelm the mind, which is unable to comprehend such vast distances. Our universe is indeed, as Flammarion expresses it, a point in the infinite. The calculations of J. E. Gore represent our highest scientific conception of the universe. He sums up his investigations with the following words: “Although we must consider the number of visible stars as strictly finite, the numbers of stars and systems really existing, but invisible to us, may be practically infinite. Could we speed our flight through space on angel wings beyond the confines of our limited universe to a distance so great that the interval which separates us from the remotest fixed star might be considered as merely a step on our celestial journey, what further creations might not then be revealed to our wondering vision? Systems of a higher order might there be unfolded to our view, compared with which the whole of our visible heavens might appear like a grain of sand on the ocean shore,—systems perhaps stretching out to infinity before us, and reaching at last the glorious ‘mansions’ of the Almighty, the Throne of the Eternal.”
CHAPTER XIII.
CELESTIAL EVOLUTION.
In the second chapter we outlined the nebular hypothesis as propounded by Herschel. Some time earlier the French mathematician, Laplace, had put forward his theory of the evolution of the Solar System. Pierre Simon Laplace was born at Beaumont-en-Auge, near Honfleur, in 1749, and was educated in the Military School of his native town. In 1767 he became Assistant Professor of Mathematics at Beaumont, and some years later at the Military School in Paris, which position he retained for many years. Member of the Institute and Minister of the Interior under Napoleon, he was created a Marquis by Louis XVIII., and died at Arcuile on March 5, 1827.
In the last chapter of his popular work, the ‘Système du Monde,’ Laplace put forward his nebular theory “with that distrust which everything ought to inspire that is not the result of observation or calculation.” Laplace noticed that in the Solar System all the planets revolved round the Sun in the same direction, from west to east, and that the satellites of the planets obeyed the same law. He also observed that the Sun, Moon, and planets rotated on their axes in the same direction as they revolved round the Sun; also that the planets moved round the Sun, and the satellites round their primaries, in almost the same plane as the Earth’s orbit, the plane of the ecliptic. It was evident that these remarkable congruities were not the result of chance, and accordingly Laplace expressed his belief that the Solar System originated from a great nebula, which in condensing detached various rings in the process of rotation. These rings condensed into the various planets and their satellites.
Laplace’s theory was powerfully supported by Herschel’s observations of the various nebulæ in the heavens. But, with the supposed resolution of the various nebulæ after the erection of the Rosse reflector in 1845, the evidence in favour of the nebular theory seemed to be greatly reduced. In 1864, however, the discovery of the gaseous nebulæ, by means of the spectroscope, gave further support to the theory. Powerful aid was lent to the nebular hypothesis by the famous German physicist, Hermann Ludwig Ferdinand von Helmholtz (1821-1894), in 1854, in his theory of the maintenance of the Sun’s heat. Many theories had been already advanced to account for this. After the discovery of the conservation of energy, Julius Robert Mayer, one of the discoverers, put forward the theory that the Solar heat was sustained by the inflow of meteorites from space, and this idea was developed in 1854 by Sir William Thomson, now Lord Kelvin (born 1824), but it was soon apparent that the supply of meteors required to sustain the Solar heat was such as would have increased the mass of the Sun very considerably. Accordingly the hypothesis was partially abandoned, and was succeeded by that of Helmholtz, who pointed out that the radiation of the Sun’s heat was the result of its contraction through cooling. The rate was then estimated at 380 feet yearly, or a second of arc in 6000 years. This theory was at once generally accepted. It assumes the Sun to be still contracting, and therefore, on going backwards in imagination, we reach a period when the Sun must have been much larger than now, and, in fact, extended beyond the orbit of Neptune.
Several objections to Laplace’s nebular theory were urged by various investigators. Among these was the retrograde motions of the satellites of Uranus and Neptune, and the extremely rapid revolution of the inner satellite of Mars. Other objections were urged by Babinet, Kirkwood, and others, and at length a sweeping reform of the nebular theory was proposed by Faye in 1884, in his work, ‘Sur l’Origine du Monde.’ Faye put forward the idea that all the planets interior to the orbit of Uranus were formed inside the solar nebula, while Uranus and Neptune came into existence after the development of the Sun was far advanced. But the objections to Faye’s theory are formidable, and the hypothesis has not been accepted.
A popular exposition of the nebular theory was given in 1901 in Ball’s work on ‘The Earth’s Beginning.’ He exhaustively discusses the whole question, and explains the retrograde motion of the satellites of Uranus and Neptune as due to the fact that the planes of the orbits of the satellites will eventually be brought to coincide with the ecliptic. These motions, says Ball, do not disprove the nebular theory. “They rather illustrate the fact that the great evolution which has wrought the Solar System into its present form has not finished its work: it is still in progress.”
The theory that the Sun’s heat was maintained by meteors, was extended by Proctor in 1870 to explain the growth of the planets through meteoric aggregation as well as nebular condensation. Certainly the theory, as developed by Proctor, accounted fairly well for the various features of the Solar System; but the highest development of the meteoritic theory is due to Lockyer, who published his views in 1890, in his work, ‘The Meteoritic Hypothesis.’ Lockyer claims that his views are merely extensions of Schiaparelli’s ideas regarding the concentration of celestial matter. He considered the chief nebular line to be identical with the remnant of the magnesium fluting, which is conspicuous in cometic and meteoric spectra; but Huggins and Keeler, with more powerful instruments, disproved the supposed coincidence. Lockyer considers that “all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat. The heat is brought about by the condensation of meteor swarms, due to gravity, the vapour being finally condensed into a solid globe.”
Lockyer divided the stars into seven groups, according to temperature, the order of evolution being from red stars through a division of second-type stars to Sirian stars, regarded as the hottest stars; through a second division of solar stars to fourth-type stars. In fact, the theory aspires to give a complete explanation of all celestial phenomena, from meteors to nebulæ. Newcomb, however, considers that the objections to the theory are insuperable, and his opinion is shared by the majority of astronomers, many of whom, however, consider that there are elements of truth in the theory; but Lockyer undoubtedly carried his ideas to an extravagant extent.
Lockyer’s evolutionary order of the stars is not supported by Vogel. Zöllner suggested in 1865 that yellow and red stars are simply white stars in a further stage of cooling; but Angström showed that atmospheric composition is a safer criterion of age than colour. Vogel’s classification, first published in 1874, and further developed in 1895, is from the standpoint of evolution. He considers Orion stars and Sirian stars to be the youngest orbs. Solar stars are considered by Vogel to have wasted much of their store of radiation, and red stars are viewed as “effete suns, hastening rapidly down the road to final extinction.” He considers stars of Secchi’s fourth type to be also dying suns, both types representing alternative roads for stars of the Solar type in their decline into dark stars. This view is supported by Dunér, and is distinctly confirmed by Hale’s observations with the Yerkes telescope. Vogel’s views, in fact, are generally accepted among astronomers. The nebular theory, modified by subsequent research, seems destined to hold its own against all attacks.
Distinctly supplementary to the nebular theory are the remarkable researches, commenced in 1879, by Sir George Howard Darwin (born 1845), son of Charles Darwin the great biologist. George Howard Darwin was born in 1845, at Downe in Kent, was educated at Cambridge, and studied for the law; but in 1873 he returned to Cambridge, where he became Plumian Professor of Astronomy in 1883. In 1879 he communicated to the Royal Society the first of his papers on tidal friction, which were summed up in his book on ‘The Tides,’ published in 1898. He finds that the tides act upon the Earth as a brake does upon a machine,—they tend to retard its rotation. Consequently, the day is growing longer, the Moon’s orbit is becoming enlarged, and its period of revolution is being lengthened.
At present the day is about twenty-four hours long, and the month about twenty-seven days. The day, however, will be lengthened at a more rapid rate than the month, and in the remote future the day and month will both last fifty-five of our present days. The Moon will revolve round the Earth in the same period that the Earth rotates on its axis, and the two bodies will perform their circuit round the Sun as if united by a bar.
Not only can we foresee the future of the Earth-Moon System, but we can also read the past. According to Darwin’s theory, the Earth, in the remote past, was probably rotating on its axis in a very short period, between three and five hours. The Moon must then have been much nearer us than it is now, and was probably revolving round its primary in the same period that the Earth took to rotate on its axis. The two globes, then gaseous, must have been revolving almost in actual contact. Had the month been even a second shorter than the day, the Moon must inevitably have fallen back on the Earth. As it was, the condition of affairs could not endure. The condition of the Moon resembled that of an egg balanced on its point. The Moon must either recede from the Earth or fall back upon it. The solar tide here interfered, and caused the Moon to recede from its primary until it reached its present distance of 239,000 miles.
The fact that the Earth and Moon were almost in contact suggests that they were probably in contact. In other words, the Moon originally formed part of the Earth, which, in consequence of its short-rotation period, and probably also owing to the interference of the solar tide, split into two portions, and the smaller of these now forms the Moon. It is likely that the matter now forming the Moon was detached from the Earth in separate particles. Just as the tides raised by the Moon tend to retard the motion of the Earth, so the Earth tides raised in the Moon have already done their work. The Moon now rotates on its axis in the same time as it revolves round the Earth. Part of the evolution of the Earth-Moon system is completed. Schiaparelli’s discovery that the rotation periods of both Venus and Mercury coincide with their times of revolution is distinctly confirmatory of Darwin’s theory.
In his chapter on the “Evolution of Celestial Systems” in his book on ‘The Tides,’ Darwin discusses the distribution of the satellites of the Solar System. He says of the evolution of a planet: “We have seen that rings should be shed from the central nucleus when the contraction of the nebula has induced a certain degree of augmentation of rotation. Now, if the rotation were retarded by some external cause, the genesis of a ring might be retarded or entirely prevented.” He then remarks that probably the formation of the Moon was retarded, and in the case of Mercury and Venus, solar tidal friction prevented satellite formation. This explains why Mercury and Venus have no satellites, the Earth only one, Mars two, while the exterior planets have each several satellites.
The theory of tidal friction was extended in 1892 to the explanation of the double stars by the American astronomer, See. See showed by mathematical calculation the effects of tidal friction in shaping the eccentric orbits of the binary stars, the course of evolution being traced from double stars, revolving almost in contact, which the spectroscope reveals, to the telescopic doubles. See’s researches have done much to supplement those of Darwin, who considers that there are two types of cosmical evolution,—the Laplacian, and the “second” or lunar type.
Lowell, in his work on ‘The Solar System’ (1903), adds six congruities to those remarked by Laplace and his successors. These are, “All the satellites turn the same face to their primaries (so far as we can judge); Mercury, and probably Venus, do the same to the Sun; one law governs position and size in the Solar System and in all the satellite systems; orbital inclinations in the satellite systems increase with distance from the primary; the outer planets show a greater tilt of axis to orbit-plane with increased distance from the Sun (so far as detectable); the inner planets show a similar relation.”
The fate of the average solar star is sketched out by Vogel’s classification, and by any evolutionary hypothesis which we may adopt. In the words of Lowell: “Though we cannot as yet review with the mind’s eye our past, we can, to an extent, foresee our future. We can with scientific confidence look forward to a time when each of the bodies composing our Solar System shall turn an unchanging face in perpetuity to the Sun. Each will then have reached the end of its evolution set in the unchanging stare of death. Then the Sun itself will go out, becoming a cold and lifeless mass; and the Solar System will circle unseen, ghostlike, in space, awaiting only the resurrection of another cosmic catastrophe.”
As to what this cosmic catastrophe will be, science gives no definite idea; nor can astronomers say with certainty whether the Universe will come to an end by the extinction of its luminaries, or whether the suns and planets will be brought back to luminosity again; but the human mind shrinks from the idea of a dead Universe. At this point science has said its last word, and must give place to religion. In our day we may repeat with deeper meaning the words of the Scottish astronomer, Thomas Dick: “Here imagination must drop its wing, since it can penetrate no further into the dominions of Him who sits on the Throne of Immensity. Overwhelmed with a view of the magnificence of the Universe, and of the perfections of its Almighty Author, we can only fall prostrate in deep humility and exclaim, ‘Great and marvellous are Thy works, Lord God Almighty.’”
INDEX.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
- A
- Absolute parallax, 158.
- Adams, J. C., 78, 116, 117, 118, 119, 120, 140.
- Adams, W., 205.
- Aerolites, 147, 148, 149.
- Airy, Sir G. B., 27, 104, 117, 120.
- Aitken, R. G., 202.
- Alcyone (η Tauri), 217.
- Aldebaran (α Tauri), 151, 166, 170, 172.
- Algol (β Persei), 178, 182, 183, 184, 193, 204.
- Al-Sufi, 180, 183.
- Altair (α Aquilæ), 170.
- Anderson, T. D., 191, 192.
- Andromeda nebula, 180, 208.
- Andromedæ (γ), 201.
- Andromedæ (Nova), 180.
- Andromedid meteors, 142, 149.
- Angström, A. J., 50, 51.
- Antares (α Scorpii), 171.
- Aquila, 195.
- Aquilæ (η), 185, 186.
- Arago, F. J. D., 6, 11, 31, 37, 40, 118, 120, 129.
- Arcturus (α Bootis), 165, 170.
- Arequipa Observatory, 75.
- Argelander, F. W. A., 27, 159, 167, 178, 179, 180, 218.
- Argo Navis, 221.
- Argus (η), 187, 188.
- Armagh Observatory, 206.
- Asteroids, 19, 62, 97-102.
- Astronomer-Royal of Scotland, 134, 155, 191;
- of England, 59, 17;
- of Ireland, 151, 156.
- Astronomy of the invisible, 203.
- Aurigæ (Nova), 191, 192, 195.
- Auwers, A., 167, 188, 203.
- B
- Babinet, 230.
- Baily, F., 159.
- Bakhuyzen, H. G., 91.
- Ball, Sir R. S., 23, 34, 108, 141, 149, 156, 158, 230.
- Barnard, E. E., 19, 95, 107, 108, 110, 111, 113, 136, 191, 211.
- Beer, W., 68, 69, 90.
- Bellatrix (γ Orionis), 209.
- Bélopolsky, A., 87, 110, 166, 185, 186, 204, 205.
- Berlin Observatory, 119, 120.
- Bessel, F. W., 24, 82, 116, 151, 152, 153, 154, 159, 167, 202, 203.
- Betelgeux (α Orionis), 165, 171, 172, 182, 187.
- Biela, W., 128.
- Biela’s comet, 128, 129, 142, 143, 146, 149.
- Birmingham, J., 189.
- Bode, J. E., 97, 98, 152.
- Bode’s Law, 97.
- Boeddicker, O., 77.
- Bond, G. P., 103, 109, 130, 136, 207.
- Bond, W. C., 109, 112, 120.
- ‘Bonn Durchmusterung,’ 159, 160, 218.
- Bonn Observatory, 88, 97, 160.
- Böotis (ε), 30.
- Borisiak, 192.
- Boss, L., 168.
- Bouvard, A., 115, 116.
- Bradley, J., 159, 167.
- Brédikhine, T. A., 105, 131, 132, 133, 134, 135.
- Brewster, Sir D., 50, 101, 178.
- Brinkley, J., 151.
- Brünnow, F., 156.
- Bruno, G., 35.
- Buffon, 103.
- Bunsen, R. W., 51.
- Burchell, 188.
- Burnham, S. W., 201, 202, 212.
- C
- Callandreau, O., 136.
- Callandrelli, 151.
- Cambridge Observatory, 116.
- Campbell, T. (Poet), 2.
- Campbell, W. W., 24, 107, 110, 166, 168, 187, 191, 193, 204, 205.
- Canals of Mars, 91, 92, 93, 94, 95.
- Cancri (ζ), 206.
- Cancri (S), 180.
- Canis Major, 188.
- ‘Cape Durchmusterung,’ 161, 162.
- Cape Observatory, 155, 157.
- Capella (α Aurigæ), 170, 176, 193, 205.
- Carnera, L., 100.
- Carpenter, J., 73.
- Carrington, R. C., 45, 46, 59.
- Cassini, D., 21.
- Cassiopeia, 221.
- Castor (α Geminorum), 30, 200, 205.
- Celoria, G., 202, 218, 221, 223, 224.
- Centauri (α), 155, 188, 225.
- Centaurus, 221.
- Cephei (δ), 178, 182, 185, 186.
- Cepheus, 221.
- Ceres, 19, 98, 101.
- Cerulli, V., 86, 91, 94.
- Chacornac, 161.
- Challis, J., 116, 119, 120.
- Chambers, G. F., 31.
- Chandler, S. C., 88, 89, 181, 184.
- Chladni, E., 138.
- Chromosphere, solar, 55, 56.
- Clark, A., 202.
- Clerke, Miss A. M., 3, 5, 8, 12, 13, 15, 25, 26, 34, 42, 58, 75, 86, 92, 105, 109, 124, 125, 131, 132, 133, 140, 142, 169, 186, 187, 189.
- Clerk-Maxwell, J., 109, 110.
- Coggia’s comet, 131, 132, 133.
- Comet families, 135.
- Comets, 24, 123-137, 141, 142, 143, 144, 146, 149, 152.
- Common, A. A., 107, 209.
- Copeland, R., 134, 135, 190, 208.
- Cornu, A., 189.
- Corona Borealis, 188.
- Corona, solar, 55, 57, 64.
- Coronæ (Nova), 188, 189.
- Crossley, E., 211.
- Crux, 221.
- Cygni (61), 152, 158.
- Cygni (Y), 184, 185.
- Cygni (Nova), 189, 190.
- Cygnus, 152, 189, 221.
- D
- Damoiseau, 78.
- D’Arrest, H. L., 96, 119, 142, 212.
- Dartmouth Observatory, 56.
- Darwin, Sir G. H., 233, 234, 235, 236.
- Dawes, W. R., 90, 117.
- De la Rue, W., 52, 75.
- Delaunay, C. E., 78, 79.
- Dembowski, E., 201.
- Deneb (α Cygni), 165.
- Denning, W. F., 84, 85, 91, 95, 105, 111, 112, 144, 145, 146.
- Deslandres, H., 110.
- Dick, T., 85, 238.
- Disc-theory, 32, 36, 38, 39, 214, 218.
- Di Vico, F., 85, 86, 170.
- Doberck, W., 201.
- Donati, G. B., 130, 131, 169.
- Donati’s comet, 130, 133, 136.
- Doppler, C., 57, 58.
- Doppler’s Principle, 58, 59, 87, 110, 165, 168, 203.
- Douglass, A. E., 92, 107.
- Draconis (λ), 182.
- Draper, H., 136, 172, 175.
- Dreyer, J. L. E., 206.
- Dunecht Observatory, 157.
- Dunér, N. C., 58, 59, 174, 175, 181, 184, 185, 201, 202, 233.
- Dunkin, E., 27, 167.
- Dunsink Observatory, 156.
- E
- Earth, 76, 97, 103, 104, 147, 148, 149, 153, 154, 156, 236.
- Earth-Moon system, 234, 235.
- Easton, C., 221.
- Eclipses, lunar, 77.
- Eclipses, solar, 56, 57, 80, 81.
- Edinburgh (Royal) Observatory, 195.
- Electrical repulsion theory, 126.
- Elger, T. G., 74.
- Elkin, W. L., 157.
- Encke, J. F., 30, 61, 119, 127, 128, 216.
- Encke’s comet, 127, 128, 137.
- Erman, 140.
- Eros, 62, 101.
- Ertborn, 85.
- Euler, L., 88, 89.
- Evolution, planetary, 228, 229, 230, 231.
- Evolution, stellar, 33, 34, 231, 232.
- F
- Faye, H., 60, 129, 230.
- Faye’s comet, 129, 137.
- Ferguson, J., 9, 178.
- Flammarion, C., 87, 91, 95, 121, 147, 164, 187, 195, 201, 202, 226.
- Flamsteed, J., 5.
- Fleming, Mrs, 192, 195.
- Forbes, G., 122.
- Fraunhofer, J. 47, 48, 49, 50, 3, 151, 153, 169.
- Fraunhofer lines, 48, 49, 50, 51, 169, 172.
- Frost, E. B., 205.
- G
- Galactic poles, 35, 224.
- Galaxies, external, 32, 218, 225, 226.
- Galaxy, or Milky Way, 32, 36-42, 186, 211, 215, 216, 217, 219, 220, 221, 224, 225.
- Galileo, 44, 107.
- Galle, J. G., 62, 108, 109, 119, 142.
- Galloway, T., 167.
- Gambart, 128.
- Gauss, C. F., 27, 98, 167.
- Gautier, A., 45.
- Gemini, 11, 194.
- Geminorum (Nova), 194.
- Geminorum (ζ), 180, 182, 185.
- George III., 11, 23.
- Gill, Sir D., 62, 136, 155, 157, 160, 161, 221.
- Glasgow Observatory, 216.
- Goodricke, J., 178, 183.
- Gore, J. E., 24, 38, 179, 181, 182, 183, 192, 202, 215, 220, 225, 226.
- Gould, B. A., 135, 160, 163, 180, 220, 221.
- Grant, R., 216.
- Gravitation, law of, 29.
- Greenwich Observatory, 59, 117.
- Grimmler, 192.
- Groombridge (1830), 156, 162, 223.
- Groombridge (1618), 156.
- Gruithuisen, 87.
- H
- Hale, G. E., 55, 57, 233.
- Hall, A., 96, 111, 112, 156, 190.
- Hall, Maxwell, 121.
- Halley, E., 138.
- Halley’s comet, 123, 130, 152.
- Halm, J., 195, 196.
- Hansen, P. A., 61, 78, 79.
- Hansky, A., 57.
- Harding, K. L., 99, 153, 220.
- Hartwig, E., 190.
- Harvard Observatory, 174, 175, 191.
- Hasselberg, B., 148, 190.
- Heis, E., 179, 220.
- Heliometer, 153, 157.
- Helium stars, 174.
- Helmholtz, H., 61, 229.
- Hencke, K. L., 99.
- Henderson, T., 154, 155.
- Henry, Paul and Prosper, 100, 114, 210.
- Hercules, 167.
- Herculis (α), 182.
- Herculis (λ), 26.
- Herschel, William, 1-42, 43, 60, 63, 65, 69, 74, 77, 85, 90, 99, 103, 109, 111, 112, 115, 123, 150, 162, 167, 176, 196, 197, 207, 214, 216, 218, 224, 227.
- Herschel, A., 144.
- Herschel, Caroline, 6, 8, 9, 12, 13, 14, 30, 35, 127, 198.
- Herschel, Sir J., 4, 17, 27, 30, 37, 50, 112, 113, 120, 130, 144, 167, 187, 188, 197, 198, 199, 200, 214, 215.
- Hind, J. R., 99, 129, 135, 180, 188.
- Hoek, 135.
- Holden, E. S., 191.
- Hough, G., 105.
- Houzeau, J. C., 220.
- Huggins, Lady, 172.
- Huggins, Sir W., 54, 57, 74, 95, 106, 114, 131, 136, 165, 170, 171, 172, 173, 189, 190, 191, 193, 195, 207, 208, 212, 231.
- Humboldt, A., 44, 139.
- Hussey, W. J., 116, 201.
- J
- Janssen, P. J. C., 52, 53, 54, 57, 59, 112, 209.
- Juno, 19, 99, 101.
- Jupiter, 20, 75, 97, 101-108, 112, 114, 121, 122, 135, 144, 146.
- Juvisy Observatory, 164.
- K
- Kaestner, 65, 124.
- Kaiser, F., 90.
- Kant, I., 34, 35, 101, 103.
- Kapteyn, J. C., 27, 158, 161, 162, 168, 221, 222.
- Keeler, J. E., 95, 114, 185, 208, 211, 212, 213, 231.
- Kelvin, Lord, 229.
- Kempf, P., 181.
- Kepler, J., 5, 35, 137.
- Kirchoff, G. R., 61, 169, 172.
- Kirkwood, D., 140, 230.
- Klein, H. J., 73.
- Klinkerfues, E., 142, 202.
- Konkoly, N., 175.
- Küstner, F., 88.
- L
- Lalande, 23, 152.
- Lambert, J. H., 34.
- Lamont, J., 44.
- Langley, S. P., 77.
- Laplace, P. S., 20, 33, 34, 77, 109, 148, 152, 195, 227, 228, 229.
- Lassell, W., 112, 115, 117, 120.
- Latitude, variation of, 88, 89.
- Leipzig Observatory, 173.
- Leonid meteors, 139, 140, 142.
- Leonis (β), 183.
- Lescarbault, 80.
- Le Verrier, U. J. J., 61, 80, 81, 118, 119, 120, 142, 164.
- Leyden Observatory, 91.
- Libræ (δ), 180.
- Lick Observatory, 93, 107, 166, 168, 191, 213.
- Light, extinction of, 40, 215, 216, 224, 225.
- Lindsay, Lord, 157.
- Linné, 71, 72.
- Lockyer, Sir J. N., 52, 53, 54, 55, 58, 149, 174, 191, 193, 195, 208, 209, 231, 232.
- Lockyer, W. J. S., 210.
- Loewy, M., 75.
- Lohrmann, W. G., 68, 71.
- Lohse, W. O., 88, 105.
- Loomis, 188.
- Lowell, P., 83, 84, 86, 87, 91, 92, 93, 94, 122, 236, 237.
- Lowell Observatory, 92, 94, 106, 114.
- Lund Observatory, 59.
- Lupus, 221.
- Luther, R., 100.
- Lyra, 221.
- Lyra (β), 178, 182, 185.
- Lyrid meteors, 122.
- M
- Maclaurin, C., 9.
- Maclear, Sir T., 155.
- Mädler, J. H., 27, 68, 69, 71, 96, 104, 202, 203, 216, 217, 218.
- Magellanic clouds, 219.
- Magnetism, 44, 60.
- Mars, 18, 19, 90-97, 101, 144, 236.
- Maunder, E. W., 59, 60, 94, 95, 134, 145, 166, 224.
- Mascari, A., 86.
- Mayer, C., 164.
- Mayer, J. R., 229.
- Mazapil meteorite, 149.
- Méchain, 127.
- Mee, A., 68.
- Melloni, 76.
- Mercury, 18, 80, 81-84, 97, 236.
- Messier, C., 30.
- Meteorites, 147, 148, 149, 229, 231.
- ‘Meteoritic Hypothesis,’ 231, 232.
- Meteors, 138-149.
- Meudon Observatory, 59.
- Milan Observatory, 82, 202, 224.
- Milky Way. See Galaxy.
- Miller, W. A., 50, 172.
- Mira Ceti, 11, 182, 186, 187.
- Mitchel, O. M., 31.
- Mizar (ζ Ursæ Majoris), 204.
- Möller, A., 129.
- Moon, the, 10, 24, 65-79, 90, 95, 148, 228.
- Moscow Observatory, 132.
- Mouchez, A., 161.
- Müller, G., 175, 181.
- Munich Observatory, 44, 224.
- N
- Napoleon, 67, 127.
- Nasmyth, J., 73, 103.
- Nebulæ, 30, 31, 207-213, 228.
- Nebular Hypothesis, 33, 195, 227, 228, 229, 230, 233.
- Neison (Nevill), E., 73.
- Neisten, 86, 105.
- Neptune, 120, 121, 135, 229, 230.
- Newall, H. F., 205.
- Newcomb, S., 27, 64, 78, 89, 94, 162, 168, 220, 222, 223, 224, 232.
- Newton, H. A., 140, 141.
- Newton, Sir I., 2, 17, 29, 77.
- Nichol, J. P., 31, 207.
- Nordvig, L., 192.
- O
- Olbers, H. W. M., 19, 20, 69, 98, 99, 123, 124, 125, 126, 127, 129, 130, 139, 148, 152, 153.
- Olbers’ comet, 125.
- Olmsted, D., 138, 207.
- Ophiuchi (α), 183.
- Orion, 221.
- Orion nebula, 10, 33, 207, 208, 209, 213.
- Orion stars, 174, 193, 209, 232.
- Orionis (κ), 209.
- Orionis (η), 209.
- Orionis (θ), 209.
- Orionis (U), 181, 182, 186, 187.
- P
- Palisa, J., 100.
- Pallas, 19, 99, 101.
- Parallax, solar, 61, 62, 63, 101.
- Parallax, stellar, 150-158, 190.
- Paris Congresses, 161.
- Paris Observatory, 78, 118, 171.
- Perrine, C. D., 81, 108, 194.
- Perrine’s comet, 136.
- Perrotin, H., 86, 91, 100, 114.
- Peck, W., 162.
- Persei (Nova), 192, 193, 194, 195.
- Perseid meteors, 122, 141.
- Perseus, 192, 221.
- Peters, C. H. F., 100.
- Peters, C. A. F., 142, 153, 155, 202.
- Photography, astronomical, 54, 56, 57, 59, 75, 81, 94, 108, 113, 136, 158, 160, 161, 172, 175, 192, 193, 194, 203, 208, 209, 210, 211, 212.
- Photometry, 176, 177.
- Piazzi, G., 19, 20, 98, 150.
- Pickering, E. C., 174, 175, 176, 177, 181, 182, 183, 185, 193, 194, 203, 204, 220, 221.
- Pickering, W. H., 75, 76, 81, 91, 92, 93, 107, 113, 209.
- Plana, G., 78, 79.
- Pleiades, 124, 210, 217.
- Pogson, N. R., 142, 180.
- Pole Star, 205.
- Pollux, 165, 170.
- Pons, J. L., 127.
- Pontécoulant, 79.
- Potsdam Observatory, 46, 173, 176.
- Pritchard, C., 158, 177.
- Proctor, R. A., 4, 20, 38, 41, 90, 91, 104, 148, 163, 164, 218, 219, 220, 231.
- Procyon (α Canis Minoris), 151, 203.
- Prominences, solar, 52, 53, 55, 64.
- Puiseux, P., 75.
- Pulkowa Observatory, 200.
- Q
- Quetelet, A., 139.
- R
- Radiant points, meteoric, 139, 144, 145, 146.
- Ranyard, A. C., 106, 146.
- Red spot on Jupiter, 105, 106.
- Regulus (α Leonis), 164, 165.
- Relative parallax, 157.
- Réseau, Photospherique, 59.
- Resisting medium, 128.
- Respighi, L., 55.
- Reversing layer, 56, 57.
- Ricco, A., 87, 105.
- Rigel (β Orionis), 165, 209.
- Ritchey, G., 194.
- Roberts, A. W., 181.
- Roberts, I., 209, 210.
- Roche, E., 109.
- Roman College Observatory, 85.
- Rosse, third Earl of, 141, 156, 207, 208, 218.
- Rosse, fourth Earl of, 77.
- Rotation of the Sun, 58, 59;
- of the planets, 82, 83, 84, 85, 86, 87, 104, 111, 112.
- Rowland, H. A., 52.
- Rutherfurd, L. M., 75, 169.
- S
- Sabine, Sir E., 44.
- Safford, T. H., 202.
- Santini, G., 159.
- Savary, F., 30, 199.
- Satellites, 96, 107, 108, 112, 113, 115, 120, 121, 236.
- Saturn, 20, 21, 22, 97, 103, 108-113, 121, 135.
- Schaeberle, J. M., 93, 107, 191, 203.
- Scheiner, C., 44.
- Scheiner, J., 166, 174, 176.
- Schiaparelli, G. V., 82, 83, 84, 85, 86, 87, 91, 92, 114, 141, 143, 149, 201, 220, 224, 231, 235.
- Schjellerup, H., 180.
- Schmidt, J. F. J., 69, 70, 71, 72, 73, 104, 179, 180, 189.
- Schönfeld, E., 160, 179, 180, 188, 189.
- Schröter, J. H., 16, 65, 66, 67, 68, 69, 70, 74, 81, 82, 84, 85, 86, 87, 97, 99, 153.
- Schwabe, S. H., 18, 43, 44, 46, 55.
- Schwassman, A., 100.
- Secchi, A., 52, 54, 55, 60, 72, 90, 114, 141, 170, 171, 173.
- Secchi’s types of stellar spectra, 170, 171, 173, 174, 175, 189, 232.
- See, T. J. J., 201, 202, 236.
- Seeliger, H., 110, 195, 196, 202, 206, 224, 225.
- Serviss, G. P., 158.
- Sirius (α Canis Majoris), 151, 170, 173, 188, 202, 225.
- Slipher, V. M., 106, 114, 204.
- Sime, J., 27.
- Sola, J. C., 112.
- Solar cluster, 221, 222.
- Solar system, motion of, 26, 27, 167, 168.
- South, Sir J., 198.
- Spectroscopic binaries, 203, 204, 205.
- Spencer, H., 218.
- Spica (α Virginis), 204.
- Spörer, F. W. G., 45, 46, 54, 59.
- Star-catalogues, 159, 160, 161, 162.
- Star-clusters, 30, 31, 32, 206, 210.
- Star-drift, 164.
- Star-gauging, 36, 40, 41, 224.
- Stars, distance of, 150-158.
- Stars, distribution of, 35, 39, 40, 198-214.
- Stars, double, 28, 29, 30, 197-206.
- Stars, gaseous, 171, 174.
- Stars, proper motion of, 162, 163, 164, 165.
- Stars, radial motion of, 165, 166.
- Stars, temporary, 156, 182, 188-196.
- Stars, triple and multiple, 206.
- Stars, variable, 177-188.
- Stellar spectra, 169-176, 187, 189, 190, 191, 193, 194.
- Stellar universe, 35-42, 214, 215-226.
- Stereo-comparator, 100, 101.
- Stokes, Sir G., 50.
- Stone, E. J., 157, 160.
- Stroobant, P., 88.
- Struve, F. G. W., 3, 37, 38, 40, 42, 128, 151, 153, 200, 214, 215, 216, 218.
- Struve, H., 201.
- Struve, L., 27, 163, 167.
- Struve, O. W., 27, 110, 115, 120, 153, 156, 163, 200, 201.
- Stumpe, O., 167.
- Sun, 15, 16, 17, 40, 43-64, 65, 80, 81, 105, 125, 128, 170, 222, 228, 229, 230, 237.
- Swift, L., 81.
- Swift’s comet, 136.
- T
- Tacchini, P., 55, 86, 87.
- Taurus, 217, 221.
- Tempel, E., 210.
- Tennyson, 96.
- Tidal friction, 79, 87, 233, 234, 235, 236.
- Tisserand, F. F., 146.
- Todd, D. P., 122.
- Trans-Neptunian planet, 121, 122.
- Trouvelot, E., 86, 87.
- Tschermak, 148.
- Tulse Hill Observatory, 171.
- Turner, H. H., 194, 224.
- Twining, A. C., 139.
- U
- Upsala Observatory, 59.
- Uranometria Argentina, 160.
- Uranus, 11, 20, 22, 23, 97, 113, 114, 115, 118, 121, 135, 141, 230.
- Ursa Major, 162, 164.
- Ursæ Majoris (δ), 182.
- Ursæ Majoris (ξ), 199.
- V
- Venus, 18, 84-88, 97, 235, 236.
- Venus, transits of, 61, 62, 87.
- Vega (α Lyræ), 151, 165, 170, 172, 173.
- Very, F. W., 77.
- Vesta, 19, 99, 101, 102.
- Vogel, H. C., 84, 88, 95, 102, 106, 114, 131, 148, 166, 173, 174, 175, 183, 184, 185, 190, 191, 193, 195, 204, 209, 232, 233, 237.
- Vulcan, 81.
- W
- Washington Observatory, 96, 223.
- Watson, J. C., 81, 100.
- Webb, T. W., 72, 73, 104.
- Weinek, L., 75.
- Weiss, E., 142.
- Well’s comet, 134.
- Whewell, W., 218.
- Williams, A. S., 110, 193.
- Wilson, A., 16.
- Winlock, J., 177.
- Winnecke, F. A. T., 131.
- Witt, K. G., 101.
- Wolf, Max, 100, 181, 191, 194, 211.
- Wolf, R., 44, 45, 188.
- Wolf and Rayet, 171.
- Wolf-Rayet stars, 171, 174.
- Wollaston, W. H., 48.
- Wright, T., 34, 110.