In 1835 Halley’s comet returned to perihelion, and was attentively studied by the most famous astronomers of the age. It was particularly studied by Sir John Herschel and by Bessel, who assisted in developing Olbers’ theory of electrical repulsion. But the most brilliant comet of the century was that which suddenly appeared on February 28, 1843, in the vicinity of the Sun. This great comet, whose centre approached the Sun within 78,000 miles, rushed past its perihelion at the speed of 366 miles a second. The comet’s tail reached the length of 200 millions of miles. The comet of 1843 was however outshone, not in brilliance but as a celestial spectacle, by the great comet discovered on June 2, 1858, byGiovanni Battista Donati(1826-1873) at Florence, and since known by his name. It became visible to the naked eye on August 19, and was telescopically observed until March 4, 1859. There was abundance of time, therefore, to study the comet, which was exhaustively observed by G. P. Bond at Harvard. His observations convinced him that the light from Donati’s comet was merely reflected sunshine, and this was generally accepted. Another great comet appeared in 1861. Like that of 1843, its appearance was sudden, being observed after sunset on June 30, 1861, when, says MissClerke, “a golden yellow planetary disc, wrapt in dense nebulosity, shone out while the June twilight in these latitudes was still in its first strength.” On the same evening the Earth and the Moon passed through the tail of the great comet. The vast majority of people never knew that such a phenomenon had taken place, and even the astronomers only noticed a singular phosphorescence in the sky—a proof of the extreme tenuity of comets.
The first application of the spectroscope to the light of comets was made by Donati in 1864. The spectrum was found to consist of three bright bands, but Donati was unable to identify them. However, his observation gave the death-blow to the theory that comets shone by reflected light alone, for it implied the existence of glowing gas in them. On the appearance in 1868 of the periodic comet discovered byFriedrich August Theodor Winnecke(1835-1897), the spectrum was examined by Huggins, who identified the bright bands with the spectrum of hydrocarbon. This was confirmed in regard to Coggia’s comet of 1874 by Huggins himself, and also Brédikhine and Vogel. The hydrocarbon spectrum is characteristic of comets, and has been recognised in all those spectroscopically studied.
The time had now come for a more completetheory of comets than that of Olbers. The theory of electrical repulsion was developed in 1871 by Zöllner, whose principle of investigation is thus described by Miss Clerke: “The efficacy of solar electrical repulsion relatively to solar attraction grows as the size of the particle diminishes.” If the particle is small enough, it will obey the repulsive, and not the attractive, power of the Sun. Zöllner considered that the smallest particles of comets obeyed the repulsive power, and thus formed the tails of comets. The development of a complete cometary theory is due, however, to the genius of a Russian astronomer. Theodor Alexandrovitch Brédikhine, born in 1831 at Nicolaieff, was employed at Moscow Observatory from 1857 to 1890, when he was promoted to the position of director at Pulkowa. He resigned in 1895, and spent his last years in St Petersburg, where he died on May 14, 1904. From the beginning of his astronomical career he was devoted to the study of comets and their tails, but it was the appearance of Coggia’s comet in 1874 which marked the commencement of his most important observations. In that year, on making certain calculations regarding the hypothetical repulsive force exerted by the Sun on various comets, he reached the conclusion that the values representingthe intensity of the repulsion fell into three classes. This was the first hint of a classification of cometary tails. Meanwhile he carefully studied the tails of comets both from direct observation and from drawings.
In 1877 he wrote: “I suspect that comets are divisible into groups, for each of which the repulsive force is perhaps the same.” Subsequent investigations led Brédikhine to divide the tails of comets into three types. The first type consists of long, straight tails, pointed directly away from the Sun, represented by the tails of the great comets of 1811, 1843, and 1861. In the second type, represented by Donati’s and Coggia’s comets, the tails, although pointed away from the Sun, appear considerably curved. In the third type the tails are, to quote Miss Clerke, “short, strongly-bent, brushlike emanations, and in bright comets seem to be only found in combination with tails of the higher classes.”
In 1879 Brédikhine fully developed his cometary theory. Assuming the reality of the repulsive force, he concluded that to produce tails of the first type, the repulsion requires to be twelve times greater than the solar attraction; the production of tails of the second type necessitates a repulsive force about equal to gravity; while the force producing third-typetails has only one-fourth the power of gravitation. It was concluded that the tails are formed by particles of matter repelled from the comet by the repulsive force of the Sun, and in tails of the first type the velocity with which these particles leave the body of the comet is four or five miles a second. Brédikhine reached the conclusion that the Sun’s repulsive force is invariable, and that the different types of tails are formed by the same force acting on different elements. The numbers 12, 1, and ¼, are inversely proportional to the atomic weights of hydrogen, hydrocarbon gas, and iron vapour. Here, then, was the key to the mystery. Brédikhine pointed out that in all probability the first-type tails are formed of hydrogen, the second of hydrocarbon, and the third of iron, with a mixture of sodium and other elements.
Within a few years of the publication of Brédikhine’s theory, five bright comets made their appearance, and there was abundant chance of testing the theory spectroscopically. In 1882 Well’s comet was particularly studied at Greenwich by Maunder, who discerned a sodium-line in its spectrum. The magnificent comet which appeared in 1882 was spectroscopically studied at Dunecht in Aberdeenshire byRalph Copeland(1837-1905), Astronomer-Royal of Scotland, whoidentified in its spectrum the prominent iron-lines as well as the sodium-line. These observations were certainly confirmatory of Brédikhine’s theory. It should also be stated, however, that several comets have shown, in addition to the hydrocarbon spectrum, that of reflected sunlight, which proves that the light we receive from comets is of a compound nature.
The comet which appeared in 1880 was announced byBenjamin Apthorp Gould(1824-1896) to be a return of the great comet of 1843. Calculations by Gould, Copeland, and Hind revealed a close similarity between the elements of the two orbits. Eventually it had to be admitted that the comets were separate bodies travelling in the same orbit. Then, two years later, the great September comet of 1882 was found to revolve in the same orbit as those of 1668, 1843, and 1880. Four years later, another comet, discovered in 1887, was found to move in the same path.
Closely allied to this subject is the existence of “comet families,” demonstrated by Hoek of Utrecht in 1865, and mentioned in our chapter on the Outer Planets. These comets are found to be dependent on the planets, Jupiter, Saturn, Uranus, and Neptune, each possessing a comet-group. Various theories have been advanced toaccount for the existence of these groups. One of these theories is that the comets have been captured by the various planets, who have forced them into their present orbits. A mathematical study byJean Pierre Octave Callandrean(1852-1904) shows that the large number of comets possessed by the various planets may be explained by the disintegration of large comets into small ones. The capture theory, it must be remembered, is purely hypothetical, and must not be regarded as anything but a theory. All that we really know is the existence of comet-families, and of comets moving in the same orbits.
The first photograph of a comet was that of Donati’s, taken in 1858 by Bond. In 1881 Tebbutt’s comet was photographed in England by Huggins, and in America byHenry Draper(1837-1882), while in 1882 Gill secured excellent photographs of the great September comet. The first photographic discovery of a comet was made by Barnard in 1892. Since then photography has been much used in cometary astronomy. No bright comets have appeared since 1882,—if we except the comet of 1901, only seen in the southern hemisphere,—although several have been just visible to the naked eye, among them Swift’s comet of 1892 and Perrine’s in the autumn of 1902. Telescopic comets, however,are very numerous, and a year never passes without one or more being discovered. The ordinary periodic comets, such as Encke’s, Faye’s, and others, are very faint, and are becoming fainter at each return—a clear proof that comets die, as Kepler said three centuries ago. This brings us to the subject of the next chapter, Meteoric Astronomy.
There is no more interesting chapter in the history of astronomy than that relating to meteors. A hundred years ago shooting-stars were not considered to be astronomical phenomena. They were supposed to be merely inflammable vapours which caught fire in the upper regions of our atmosphere, although both Halley and the scientistErnst Chladni(1756-1827) had notions of their celestial origin. For thirty-three years after the beginning of the century, however, nothing was heard of meteoric astronomy, nor was the subject considered as part of the astronomer’s labours.
A great meteoric shower took place on the night of November 12 and morning of November 13, 1833. The shower was probably the grandest ever witnessed, the shooting-stars being literally innumerable. The display was best observed in America, and was attentivelywatched byDenison Olmsted(1791-1859), Professor of Mathematics at Yale, and by the American physicist,A. C. Twining(1801-1884). These investigators discovered that all the meteors which fell during the great shower seemed to come from the same part of the celestial vault. In other words, their paths, when traced back, were found to converge to a point near the star γ Leonis. This observation gave the death-blow to the theory of their terrestrial origin. The point known as the “radiant” was clearly a point independent of the Earth. Olmsted also recognised the fact that the shower had taken place in the previous year, and he regarded it as produced by a swarm of particles moving round the Sun in a period of 182 days. Soon after this it was noticed that the phenomenon took place in 1834 and subsequent years with gradually decreasing intensity. It was then remembered that Humboldt had observed in November 1799 a very brilliant shower, and accordingly Olbers suggested that another shower might be seen in 1867.
The falling stars of August were next proved byAdolphe Quetelet(1791-1874) to form another meteoric system; and accordingly the theory of Olmsted that the November meteors moved round the Sun in 182 days had to be abandoned,for, says Miss Clerke, “If it would be a violation of probability to attribute toonesuch agglomeration a period of an exact year or sub-multiple of a year, it would be plainly absurd to suppose the movements oftwoor more regulated by such highly artificial conditions.” Accordingly Erman suggested in 1839 the theory that meteors revolved in closed rings, intersecting the terrestrial orbit; and that when the Earth crossed through the point of intersection, it met some members of the swarm. The subject now remained in abeyance for thirty-four years, if we except some wonderful ideas put forward in 1861 byDaniel Kirkwood(1813-1896), an American astronomer, who stated his belief in the disintegration of comets into meteors; but little attention was paid to his opinions. In 1864 the subject was taken up byHubert Anson Newton(1830-1896), Professor at Yale, who undertook a search through ancient records for the thirty-three-year period of the Leonids or November meteors. His search was highly successful, and having demonstrated the existence of the period, Newton set himself to determine the orbit. He indicated five possible orbits for the swarm, ranging from 33 years to 354½ days. Newton was unable to solve the question mathematically; but here Adams, the discoverer of Neptune, came to therescue, and demonstrated that the period of 33¼ years was alone possible, and that the others were untenable. These investigations, completed in March 1867, proved the existence of a great meteoric orbit extending to the orbit of Uranus.
Meanwhile Newton had predicted a meteoric shower on the evening of November 13 and morning of November 14, 1866. His prediction was fulfilled. The shower was inferior to that of 1833, but was still a magnificent spectacle. Sir Robert Ball, then employed at Lord Rosse’s Observatory, observed the shower, and records the impossibility of counting the meteors. This great shower attracted the attention of astronomers all over the world to the study of meteors. Meanwhile Schiaparelli had been working at the subject for some time, and in four letters addressed to Secchi, towards the end of 1866, he showed that meteors were members of the Solar System, possessed of a greater velocity than that of the Earth, and travelling in orbits resembling those of comets, in the fact that they moved in no particular plane, and that their motion was both direct and retrograde. Schiaparelli computed the orbit of the Perseids or August meteors, and was astonished to find it identical with the comet of August 1862. This was a proof of the connection between these two apparently widelydifferent types of celestial bodies. Early in 1867 Schiaparelli found that Le Verrier’s elements for the orbit of the Leonids were identical with those of the comet of 1866, discovered byErnst Tempel(1821-1889). Peters of Altona had meanwhile reached the same conclusion; whileEdmund Weiss(born 1837) of Vienna pointed out the similarity of the orbit of a star-shower on April 20 and that of the comet of 1861. He also drew attention, independently of Galle and D’Arrest, to the close connection between the orbits of the lost Biela’s comet and the Andromedid meteors of November.
All doubt as to the connection of comets and meteors was removed by the great shower on November 27, 1872. Biela’s lost comet was due at perihelion in 1872, and although searched for was not observed; but when the Earth crossed its orbit, a great meteoric shower took place. “It became evident,” says Miss Clerke, “that Biela’s comet was shedding over us the pulverised products of its disintegration.” The shower was little inferior to that of 1866. MeanwhileErnst Klinkerfues(1827-1884), Professor at Göttingen, believing that Biela’s comet itself had encountered the Earth, telegraphed toNorman Robert Pogson(1829-1891), Government astronomer at Madras, to search for the comet in theopposite region of the sky. Pogson did observe a comet, but certainly not Biela’s, although probably another fragment of the missing body.
The theory of the actual disintegration of comets was enunciated by Schiaparelli in 1873, and developed in his work ‘Le Stelle Cadenti.’ He was led to regard comets as cosmical clouds formed in space by “the local concentration of celestial matter.” He then remarks that a cosmical cloud seldom penetrates to the interior of the Solar System, “unless it has been transformed into a parabolic current,” which may occupy years, or centuries, in passing its perihelion, “forming in space a river, whose transverse dimensions are very small with respect to its length: of such currents, those which are encountered by the earth in its annual motion are rendered visible to us under the form of showers of meteors diverging from a certain radiant.”
Schiaparelli next pointed out that when the current of meteors encounters a planet, the resulting perturbations cause some of the meteoric bodies to move in separate orbits, forming the bolides and aerolites which fall from the sky at intervals. “The termfalling stars” he says, “expresses simply and precisely the truth respecting them. These bodies have the samerelation to comets that the small planets between Mars and Jupiter have to the larger planets.” In the third chapter of his ‘Le Stelle Cadenti’ he explicitly states that “the meteoric currents are the products of the dissolution of comets, and consist of minute particles which certain comets have abandoned along their orbits, by reason of the disintegrating force which the Sun and planets exert on the rare materials of which they are composed.”
In 1878Alexander Stewart Herschel(born 1836), son of Sir John Herschel, and a famous meteoric observer, published a list of known or suspected coincidences of meteoric and cometary orbits, amounting to seventy-six. Meanwhile much progress has since been made in the observation of meteoric showers and the determination of their radiant points. In this branch of astronomy, by far the greatest name is that of William Frederick Denning, the self-made English astronomer. Born at Redpost, in Somerset, in 1848, his career of meteoric observation commenced in 1866. For the past forty years he has attentively devoted himself to the observation of meteors. From 1872 to 1903 he determined the radiant points of no fewer than 1179 meteoric showers. In addition to this, he published, in 1899, acatalogue of meteoric radiants, containing 4367; and he has carefully studied the remarkable objects known as fireballs or “sporadic meteors.” He has occasionally been able to trace a connection between fireballs and weak meteoric showers, but he concludes that they “must either be merely single sporadic bodies, or else the survivors of some meteor group, nearly exhausted by the waste of its material during many past ages.” All of Denning’s meteoric work has been done in his spare time, for it must be borne in mind that he pursues the profession of accountant in Bristol, and that only his leisure hours have been devoted to the science of astronomy. His researches have been entirely conducted with the unaided eye. His only instrument is a perfectly straight wand, which he uses as a help and corrective to the eye in ascribing the paths of the meteors. Thanks to the laborious work of this able English astronomer, the observation of meteors is now ascientificbranch of astronomy. In the words of Maunder, “for six thousand years men stared at meteors and learned nothing, for sixty years they have studied them and learned much, and half of what we know has been taught us in half that time by the efforts of a single observer.”
Further meteoric showers from Biela’s comet were seen in 1885 and 1892. The Leonid shower was confidently predicted for 1899, in accordance with the thirty-three-year period, but the great display did not come off, either in 1899 or 1900. In 1901 there was a certain weak shower observed in America; and similar displays took place in 1903 and 1904. Many explanations have been given as to the failure of the shower, the most probable idea being that the attraction of Jupiter diverted the meteors from their course.
Denning’s observations on meteors resulted, as early as 1877, in the discovery of so-called “stationary radiants.” The radiant-point of a long enduring shower usually exhibits an apparent motion, resulting from the combined orbital motions of the Earth and the meteors; but Denning found that in some cases the shower, though lasting for months, persistently exhibited the same radiant-point, implying that the motion of the Earth must be insignificant compared with that of the meteors, computed by Ranyard at 880 miles per second. The difficulty of admitting so great a velocity led the French astronomer,François Felix Tisserand(1845-1896), to doubt the existence of these stationary radiants; but the fact of theirexistence cannot be doubted, although no really satisfactory explanation has been offered.
Another type of meteors comprises the bodies termed respectively as bolides, uranoliths, and aerolites,—stones which fall to the Earth from the sky. In 1800 the French Academy declared the accounts of stones having fallen from the heavens to be absolutely untrue. Three years later an aerolite fell at Laigle, in the Department of Orne, on April 26, 1803, attended by a terrific explosion. In the words of Flammarion, “Numerous witnesses affirmed that some minutes after the appearance of a great bolide, moving from south-east to north-east, and which had been perceived at Alençon, Caen, and Falaise, a fearful explosion, followed by detonations like the report of cannon and the fire of musketry, proceeded from an isolated black cloud in a very clear sky. A great number of meteoric stones were then precipitated on the surface of the ground, where they were collected, still smoking, over an extent of country which measured no less than seven miles in length.”
Some aerolites, instead of being shattered into fragments, have been observed to fall to the Earth intact, and bury themselves in the ground. Numerous instances have been observed during the last century, and masses of meteoric stoneshave been found in positions which clearly indicate that they must have fallen from the sky. Chemists have made analyses of the elements in these remarkable bodies, and have found them to contain iron, magnesium, silicon, oxygen, nickel, cobalt, tin, copper, &c. The spectrum of these aerolites, raised to incandescence, has been studied by Vogel and by the Swedish observer,Bernhardt Hasselberg(born 1848), who detected the presence of hydrocarbons, which are also present in cometary spectra.
When the existence of aerolites as celestial bodies was first recognised, Laplace suggested that they had been ejected from volcanoes on the Moon. This theory, although supported by Olbers and other astronomers, was soon rejected. Next, it was suggested that they were ejected from the Sun, and Proctor believed them to come from the giant planets. A very detailed discussion of the subject is to be found in Ball’s ‘Story of the Heavens’ (1886), in which he expresses views in harmony with those of the Austrian physicist Tschermak. Ball demonstrated that the meteors which fall to the Earth cannot have come from any other planet, nor from the Sun. Accordingly, he concluded that they were originally ejected by the volcanoes of the Earth many ages ago, when they wereactive enough to throw up pieces of matter with a velocity great enough to carry them away from the Earth altogether. Such meteors would, however, intersect the terrestrial orbit at each revolution.
The alternative theory to this, supported by Schiaparelli and Lockyer, is that the aerolites are merely larger members of the meteor-swarms, which have been deflected from their paths. The chief objection to this theory is the absence of connection between the meteoric showers and the falls of aerolites and bolides. Only on one occasion was a meteoric stone observed to fall during a shower. On November 27, 1885, during the shower of Andromedid meteors from Biela’s comet, a large bolide, weighing more than eight pounds, fell at Mazapil, in Mexico. This, however, was the only case hitherto observed; and it may have been merely a coincidence.
The most remarkable progress in astronomy during the past century has been in the department of sidereal science, or the study of the Suns of space, observed for their own sakes, and not merely for the purpose of determining the positions of the Sun and Moon, and to assist navigation. Thanks to Herschel, the nineteenth century witnessed the steady development of stellar astronomy, combined with many important discoveries and investigations.
The one pre-Herschelian problem in sidereal astronomy was the distance of the stars. Owing to its bearing on the Copernican theory, the problem was attacked by the astronomers of the seventeenth and eighteenth centuries. Herschel made numerous attempts to detect the parallax of the brighter stars, but failed. Meanwhile there had been many illusions. Piazzi believed that his instruments—which in reality wereworn out and unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and Vega; Calandrelli, another Italian, andJohn Brinkley(1763-1835), Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was shown byFriedrich Georg Wilhelm Struve(1793-1864), the great German astronomer, that no instruments then in use could possibly be successful in measuring the stellar parallax. A few years later, however, Fraunhofer brought the refractor to a degree of perfection surpassing all previous efforts. In 1829 he mounted for the observatory at Königsberg a heliometer, the object-glass of which was divided in two, and capable of very accurate measurements. This heliometer eventually revealed the parallax of the stars in the able hands of Friedrich Wilhelm Bessel.
Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of Hanover, on July 22, 1784. His father was an obscure Government official, unable to provide a university education for his son. Bessel’s love of figures, together with an aversion to Latin, led him to pursue a commercial career. At the age of fourteen, therefore, he entered as an apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was not content, however, to remain in that humble position. His greatambition was to become supercargo on one of the trading expeditions sent to China; and so he learned English, Spanish, and geography. But he never became a supercargo. In order to be fully equipped for such a position, he determined to learn how to take observations at sea, and his acquaintance with observation aroused a desire to study astronomy. He constructed for himself a sextant, and by means of this, along with a common clock, he determined the longitude of Bremen.
Such enthusiasm could not be long without its reward. For several years Bessel remained a clerk, and the hours devoted to study were those spared from sleep. He studied the works of Bode, Von Zach, Lalande, and Laplace, and in two years was able to compute the orbits of comets by means of mathematics. From some observations of Halley’s comet at its appearance in 1607, Bessel calculated its orbit, and forwarded the calculation to Olbers, then the greatest authority on cometary astronomy. Olbers was delighted at this work, and he sent the results to Von Zach, who published them. The self-taught young astronomer had accomplished a piece of work which fifteen years before had taxed the skill and patience of the French Academy of Sciences.
In 1805, Harding, Schröter’s assistant at Lilienthal, resigned his position for a more promising one at Göttingen. Olbers procured for Bessel the offer of the vacant post, which the latter accepted. At Lilienthal Bessel received his training as a practical astronomer. He remained in Schröter’s observatory until 1809. Although only twenty-five years of age, he had become so well known in Germany that in that year he was appointed Professor of Astronomy in the University of Königsberg, and was chosen to superintend the erection of the new observatory there. Within a few years a clerk in a commercial office had worked his way from obscurity to fame.
In 1813 the Königsberg Observatory was completed, and here Bessel worked for thirty-three years, until his death, on March 17, 1846. It was only about ten years before his death that he commenced his search for the stellar parallax, with the aid of Fraunhofer’s magnificent heliometer. He determined to make a series of measures on a small double star of the fifth magnitude in the constellation Cygnus, named 61 Cygni, the large proper motion of which led him to suspect its proximity to the Solar System. From August 1837 to September 1838 he made observations on 61 Cygni, and he found thatthere was an annual displacement which could only be attributed to parallax. In order to have no mistake, he made another year’s observations, which confirmed the results he arrived at previously, and all doubt was removed by a third series. The resulting parallax was 0·3483″, corresponding to a distance of 600,000 times the Earth’s distance from the Sun. This was confirmed some years later by C. A. F. Peters at Pulkowa, and still later by Otto Struve, who estimated the distance at forty billions of miles. Meanwhile, F. G. W. Struve, working at Pulkowa, found a parallax of 0·2613″ for Vega, but this was afterwards found to be considerably in error. Accordingly, Struve does not rank with Bessel as a successful measurer of star-distance. But independently of Bessel, another accurate measure had been made byThomas Henderson, the great Scottish astronomer.
Born in Dundee in 1798, Thomas Henderson was the youngest of five children of a hard-working tradesman. After education in his native town he went to Edinburgh, where he worked for years as an advocate’s clerk, pursuing studies in astronomy as a recreation from his boyhood. In 1831 he had become so well known, that he received the appointment of Astronomer-Royal at the new observatory atthe Cape of Good Hope. But the climate of South Africa did not suit his health, and after a year he returned to Scotland. In 1834 he became Professor of Astronomy in the University of Edinburgh, and Astronomer-Royal of Scotland, which position he held till his death on November 23, 1844, at the early age of forty-six.
During a year’s work at the Cape, Henderson undertook a series of observations on the bright southern star, α Centauri, with a view to determining its parallax. These observations were made in 1832 and 1833, but were not reduced until Henderson’s return to Scotland. At length, on January 3, 1839, he announced to the Royal Astronomical Society that he had succeeded in measuring the parallax of α Centauri, which he determined as about one second of arc, corresponding to a distance of about twenty billions of miles. This result was confirmed by the observations ofThomas Maclear(1794-1879), his successor at the Cape, and by those of later observers, notably Sir David Gill, who has reduced the parallax to 0·75″.
Other determinations of stellar parallax, some genuine and others illusory, were made soon after these successful observations. C. A. F. Petersand Otto Struve at Pulkowa were among the most famous parallax-hunters in the middle of the century. One of the most successful searchers after parallax was the German astronomerFriedrich Brünnow(1821-1891), who was employed from 1865 to 1874 as Astronomer-Royal of Ireland. He determined the parallax of Vega as 0·13″, and this was confirmed in 1886 by Hall at Washington: while he measured the parallax of the star Groombridge 1830, which turned out to be 0·09″. He resigned his post in 1874, and his successor at Dublin Observatory proved to be his successor also in this branch of astronomy.Robert Stawell Ball, born in Dublin in 1840, was astronomer to Lord Rosse in 1865 and 1866, and became in 1874 Astronomer-Royal of Ireland in succession to Brünnow, a position which he filled until his appointment in 1892 as Professor of Astronomy at Cambridge, and director of the observatory there. During his term of office in Dublin he undertook, in 1881, a “sweeping search” for large parallaxes, thereby disproving certain ideas as to the proximity to the Earth of red and temporary stars; while he also determined the parallax of the star 1618 Groombridge.
But the greatest extension of our knowledge of stellar distances, in recent years, is due to aScottish astronomer, who has maintained the reputation of Scotland, and also of the Cape Observatory, in this line of research. Born in Aberdeen in 1843,David Gilldirected Lord Lindsay’s private observatory at Dunecht, in Aberdeenshire, from 1876 to 1879. In the latter year he succeededEdward James Stone(1831-1897) as Astronomer-Royal at the Cape, a position which he has since filled with conspicuous ability. From 1881 he has been engaged in the hunt for parallax. In conjunction withWilliam Lewis Elkin(born 1855), now director of Yale College Observatory, he determined the parallaxes of nine stars with the aid of Lord Lindsay’s heliometer. In 1887, with a larger instrument, he resumed the search, while Elkin worked in co-operation with him, but at Yale Observatory, where he undertook the measurement of the parallaxes of northern stars. He fixed in 1888 an average parallax for first-magnitude stars, which was determined at 0·089″, corresponding to a journey for light of thirty-six years.
Most of the successful determinations of parallax have been made by the “relative” method—that is, the determination of the displacement of a star in reference to another star, assumed to be situated at an immeasurable distance.The method of absolute parallax, on the other hand,—the star’s displacement in right ascension and declination,—has been seldom used, owing to the laborious reduction which has to be gone through before the result can be reached. In 1885, however, a series of observations were undertaken at Leyden byJacobus Cornelius Kapteyn(born 1851), who determined by the absolute method the parallaxes of fifteen northern stars.
The first application of photography to the problem was due to the zeal and energy ofCharles Pritchard(1808-1893), Professor of Astronomy at Oxford, who determined by this method the parallax of 61 Cygni, which he announced in 1886 to be 0·438″, in agreement with Ball’s determination. He also determined the average parallax of second-magnitude stars, which came out as 0·056″. Since the time of Pritchard’s observations various other more or less satisfactory determinations of parallax have been made. Few of the parallax determinations are probably very accurate, and none exact; but an idea of the difficulty of the measurement may be gathered from the remark of an American writer, Mr G. P. Serviss, that the displacement “is about equal to the apparent distance between the heads of two pins, placed an inch apart, andviewed from a distance of a hundred and eighty miles.”
Closely allied to the question of parallax is the determination of the exact positions of the stars and the formation of star-catalogues. In this branch, too, much is due to the genius of Bessel. The observations of Bradley at Greenwich from 1750 to 1762 were reduced by Bessel into the form of a catalogue, which was published in 1818, with the title of ‘Fundamenta Astronomiæ.’ During the years 1821 to 1823 Bessel took 75,011 observations, by which he brought up the number of accurately known stars to 50,000. At the same time notable catalogues had been constructed, particularly by the English astronomer,Francis Baily(1774-1844), and byGiovanni Santini(1786-1877), director of the observatory at Padua; but Bessel’s successor in this branch of research wasFriedrich Wilhelm August Argelander(1799-1875). In 1821 he became assistant to Bessel at Königsberg, in 1823 director of the Observatory at Abo, in Finland, and in 1837 of that at Bonn. Here he commenced in 1852 the great ‘Bonn Durchmusterung,’ a catalogue and atlas of 324,198 stars visible in the northern hemisphere. The great catalogue was published in 1863. After Argelander’s death it was extended so as toinclude 133,659 stars in the southern hemisphere, by his assistantEduard Schönfeld(1828-1891), who succeeded him in 1875 as director of Bonn Observatory, where he died in 1891. Meanwhile a greater undertaking was commenced in 1865 by the Astronomische Gesellschaft. This was the co-operation of thirteen observatories in Europe and America for the exact determination of the places of 100,000 of Argelander’s stars.
In the southern hemisphere, working at Cordova in Argentina, was the great American astronomer,Gould, whose ‘Uranometria Argentina,’ published in 1879, gives the magnitudes of 8198 stars, and whose Argentine General Catalogue, containing reference of 32,448 stars, was published in 1886. The late Radcliffe observer, Stone, published a useful catalogue in 1880 from his observations at the Cape.
The application of photography to the work of star-charting dates from 1882, when Gill photographed the comet of 1882, and was struck with the distinctness of the stars on the background. For some time he had contemplated the extension of the ‘Durchmusterung,’ from the point where Schönfeld left it, to the southern pole, and the idea struck him to utilise photography for the purpose. In 1885, accordingly, Gill commenced work, and in four years all thephotographs were taken. The reduction of the observations into the form of a catalogue was spontaneously undertaken by the great Dutch astronomer, Kapteyn, who was occupied with the work for fourteen years, until in 1900 the great catalogue, known as the ‘Cape Photographic Durchmusterung,’ was completed. Half a million stars are represented on the plates taken at the Cape.
By the time the ‘Durchmusterung’ was completed, a greater undertaking was in progress. Paul and Prosper Henry, astronomers at the Paris Observatory, when engaged in continuing Chacornac’s ecliptic charts, applied photography to their work, and found it very successful. Accordingly Gill’s proposal, on June 4, 1886, of an International Congress of Astronomers, to undertake a photographic survey of the heavens, was enthusiastically received by the French astronomers. The Congress met at Paris in 1887, under the presidentship ofAmédée Mouchez(1821-1892), director of the Paris Observatory, fifty-six astronomers of all nations being present. The Congress resolved to construct a Photographic Chart, and a Catalogue, the former containing twenty million stars, the latter a million and a quarter. Meetings were held in Paris in 1891, 1893, 1896, and 1900 to superintendthe progress of the work, which is now (1906) well advanced towards completion.
A unique star catalogue is in course of preparation by the Scottish astronomer,William Peck(born 1862), astronomer to the City of Edinburgh since 1889. Mr Peck’s catalogue is accompanied by a series of charts. His star-magnitudes are those of all famous catalogues reduced to a standard scale. This catalogue, the result of more than fifteen years’ work, will be an important addition to the many valuable works of the kind already in existence, and will further increase the already great reputation of Scotsmen in practical astronomy.
The determination of the proper motions of the stars is another important branch of practical astronomy in which much progress has been made since the time of Herschel. Stars with much larger proper motions than those of the first magnitude have been discovered. For many years the small sixth-magnitude star in Ursa Major, 1830 Groombridge, was supposed to be the swiftest of the stars, and was named by Newcomb the “runaway star.” But in 1897, on examining the plates of the ‘Cape Durchmusterung,’ Kapteyn discovered a still swifter star of the eighth magnitude, situated in the southern constellation, Pictor. The rateof its motion is over eight seconds of arc yearly; and an idea of the vast distance of the stars may be obtained by the statement that it would take 200 years for the star—known as Gould’s Cordova Zones, V Hour 243—to move over a space equal to the moon’s diameter. Important observations have been made on the stellar motions, and on their bearing on the structure of the Universe, by various astronomers, including J. C. Kapteyn andLudwig Struve(born 1858), son of Otto Struve; but these must be reserved for a later chapter.
Richard Anthony Proctor, born at Chelsea, in London, in 1837, graduated at Cambridge in 1860. For the next twenty-eight years he earned his living by publishing many volumes on astronomy, popular and technical, fifty-seven having appeared at the time of his death, which took place at New York on September 12, 1888. Notwithstanding the vast amount of work bestowed on his books, his original investigations were permanent contributions to astronomical science. In 1870 he undertook to chart the directions and amounts of 1600 proper motions. While engaged on this work, it occurred to him that it would be “desirable and useful to search for subordinate laws of motion.” He found, from the laborious process of charting, that five ofthe seven stars of the Plough had a motion in common—that is to say, were moving in the same direction at the same rate. This phenomenon was termed by Proctor “star-drift.” He also recognised other instances of star-drift in other portions of the heavens.
The subject was soon afterwards taken up by the French astronomer, Camille Flammarion. Born in 1842 at Montigny-le-Roi, in Haute Marne, Flammarion was appointed assistant to Le Verrier in 1858, but gave up his post in 1862. Employed successively at the Bureau des Longitudes, and as editor of scientific papers, he founded in 1882 his private observatory at Juvisy-sur-Orge, where he has since continued his investigations.
Following up Proctor’s discovery of star-drift, Flammarion drew charts of proper motions. He demonstrated the “common proper motion” of Regulus and an eighth-magnitude star, Lalande 19,749, from a comparison of his measures in 1877 with those of Christian Mayer a century previously; while he discovered many other instances. His reflections on these motions, as given in his ‘Popular Astronomy,’ are worthy of reproduction: “Such are the stupendous motions which carry every sun, every system, every world, all life, and all destiny in alldirections of the infinite immensity, through the boundless, bottomless abyss; in a void for ever open, ever yawning, ever black, and ever unfathomable; during an eternity, without days, without years, without centuries, or measures. Such is the aspect, grand, splendid, and sublime, of the universe which flies through space before the dazzled and stupefied gaze of the terrestrial astronomer, born to-day to die to-morrow, on a globule lost in the infinite night.”
Measures of proper motion only enable us to determine the motion of stars across the line of sight. They do not tell us whether the star is advancing or receding. Here, however, the spectroscope comes to our aid by means of Doppler’s principle, described in the chapter on the Sun. It occurred to Huggins that, by observing the displacement of the lines in the spectra of the stars, he could determine their motion in the line of sight. His first results were announced in 1868. In the case of Sirius, the displacement of the line marked F was believed to indicate a velocity of recession of 29 miles a second. Some time later Huggins announced that Betelgeux, Rigel, Castor, and Regulus were retreating, while Arcturus, Pollux, Vega, and Deneb were approaching. Soon after this successful work thesubject was taken up by Maunder at Greenwich and by Vogel at Bothkamp; but the delicacy of the measurements prevented satisfactory results from being reached through visual observations, and accordingly the measurements were very discordant.
In 1887 H. C. Vogel, working at Potsdam Astrophysical Observatory, applied photography to the measurement of radial motion. Assisted byJulius Scheiner(born 1858), he determined the radial motions of fifty-one bright stars by photographing the stellar spectra and measuring the photographs. Vogel found 10 miles a second to be the average velocity of stars in the line of sight, the tendency of the eye being to exaggerate the displacements. The swiftest of the stars measured by Vogel proved to be Aldebaran, with a velocity of recession of 30 miles a second. Since 1892 the subject has been pursued by Vogel himself with the new 30-inch refractor at Potsdam, by Campbell at the Lick Observatory, Bélopolsky at Pulkowa, and other observers. Towards the end of 1896 Campbell undertook, with the 36-inch Lick refractor, a series of measures on radial motion, and many important discoveries were made. These, however, must be reserved for the chapter dealing with double stars.
Herschel’s great discovery, from the apparent motions of the stars, of the movement of the Solar System was not accepted by the next generation of astronomers. Bessel declared in 1818 that there was absolutely no evidence to show that the Sun was moving towards Hercules. Even Sir John Herschel rejected his father’s views, although some confirmatory results had been reached by Gauss. At length, in 1837, Argelander, in a memorable paper, based on his observations at Abo, in Finland, attacked the problem, and demonstrated, from a discussion of the motions of 390 stars, quite independently of Herschel’s work, that the Solar System was moving towards Hercules. This was confirmed in 1841 by Otto Struve, in 1847 byThomas Galloway, and in 1859 and 1863 by Airy andEdwin Dunkin(1821-1898), assistant at Greenwich Observatory.
Meanwhile, in 1886,Arthur Auwers, permanent Secretary of the Berlin Academy of Sciences, completed the re-reduction of Bradley’s observations at Greenwich, and brought out 300 reliable proper motions, which were utilised by Ludwig Struve, whose investigation removed the solar apex from Hercules to the neighbouring constellation Lyra: this slight change was confirmed byOscar Stumpe, of Bonn, andLewisBoss(born 1847), director of the Observatory at Albany, New York. An investigation by Newcomb fully confirmed the previous results. In 1900, 1901, and 1902 Kapteyn made three distinct investigations on the solar motion, and still further confirmed the previous investigations.
These investigations are fully confirmed by the application to the question of Doppler’s principle of measuring radial motion. The spectroscopic researches of Campbell at the Lick Observatory place the solar apex very near the position assigned to it by Newcomb and Kapteyn. Campbell finds the solar velocity to be about 12 miles a second, and Kapteyn thinks a velocity of about 11 miles a second is “the most probable value that can at present be adopted.”