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 that there 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 by Thomas 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 at the 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, a 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 a 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 of Thomas 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. Peters and 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 astronomer Friedrich 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 a Scottish 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 Gill directed Lord Lindsay’s private observatory at Dunecht, in Aberdeenshire, from 1876 to 1879. In the latter year he succeeded Edward 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 with William 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 by Jacobus 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 of Charles 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, and viewed 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 by Giovanni Santini (1786-1877), director of the observatory at Padua; but Bessel’s successor in this branch of research was Friedrich 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 to include 133,659 stars in the southern hemisphere, by his assistant Eduard 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 the photographs 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 of AmÉdÉe 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 superintend the 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 rate of 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 and Ludwig 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 of the 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 all directions 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 the subject 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 by Julius 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.