A Century's Progress in Astronomy

WILLIAM BLACKWOOD AND SONS EDINBURGH AND LONDON MCMVI All Rights reserved

PREFACE.

The present volume originated in a desire to present, in small compass, a record of the marvellous progress in astronomy during the past hundred years. Indebtedness should be acknowledged to the valuable works of Professor Newcomb, Professor Schiaparelli, Professor Lowell, Professor Young, Sir Robert Ball, Mr Gore, M. Flammarion, and Miss Clerke, who, as the historian of modern astronomy, occupies a place at once authoritative and unique.

Portions of Chapters II. and XII. have already appeared in the form of an article on the Construction of the Heavens, contributed by the writer to the American periodical, ‘Popular Astronomy.’

Balerno, Mid-Lothian, October 1906.

CONTENTS.

PAGE CHAPTER I. 1 HERSCHEL THE PIONEER. Influence of Herschel’s work—His characteristics—Birth and early years—Emigration to England—Caroline Herschel—Discovery of Uranus—King’s Astronomer—Latter years and death—Death of Caroline Herschel CHAPTER II. 15 HERSCHEL THE DISCOVERER. Solar researches—Study of Venus—Of Mars—The Asteroids—Jupiter—Saturn—Discovery of satellites—Uranian satellites—Cometary researches—Motion of the Solar System—Discovery of binary stars—Clusters and nebulæ—Nebulous stars—The Nebular Hypothesis—Star-gauging—The disc-theory—Subordinate clusters—Abandonment of the disc-theory—Second method of star-gauging—Estimate of Herschel’s work CHAPTER III. 43 THE SUN. Schwabe and the sun-spot period—Researches of Wolf, Lamont, Sabine, Gautier—Observations of Carrington and Spörer—Career and work of Fraunhofer—Spectrum analysis—Work of Kirchhoff—Solar eclipse work—The Solar prominences—Janssen and Lockyer—Huggins and Zöllner—Work of Young—The Italian spectroscopists, Secchi, Respighi, Tacchini—Career of Tacchini—The reversing layer—The Corona—Doppler’s principle—Rotation of the Sun—Work of Dunér—Janssen’s solar atlas—Maunder and magnetism—Solar theories—Distance of the Sun—Summary CHAPTER IV. 65 THE MOON. Life and work of Schröter—Of Mädler—Of Schmidt—Changes on the Moon—Selenography in England—Lunar atmosphere—Lunar photography—Work of W. H. Pickering—The new Selenography—The Moon’s heat—Motion of the Moon—Acceleration of the Moon’s mean motion—Work of Laplace, Adams, Delaunay CHAPTER V. 80 THE INNER PLANETS. The problem of Vulcan—Mercury—Work of Schröter—Schiaparelli, his life and work—Work of Lowell—Spectrum of Mercury—Venus—Rotation period: work of Schröter, Di Vico, Schiaparelli, Tacchini, Lowell—Atmosphere and surface of Venus—The Earth: variation of latitude—Mars—Rotation of Mars—Surface—Discovery of canals—Work of Schiaparelli and Lowell—Interpretation of the canals—The theory of intelligent life—Spectrum of Mars—Satellites—The Asteroids—Bode’s law—Work of Piazzi and Olbers—Application of photography by Wolf—Discovery of Eros CHAPTER VI. 103 THE OUTER PLANETS. Physical condition of Jupiter—Work of Zöllner and Proctor—The red spot—Satellites—Discovery of fifth satellite—Sixth and seventh satellites—Rings of Saturn: Bond, Maxwell, Keeler—Struve’s theory—Globe of Saturn—New satellites—Uranus and its satellites—Discovery of Neptune—Adams and Le Verrier—Satellite—Trans-Neptunian planets CHAPTER VII. 123 COMETS. Life and work of Olbers—His repulsion theory—Life and work of Encke—His comet—Biela’s comet—Faye’s comet—Return of Halley’s comet—Donati’s comet—Comet of 1861—Spectroscopic study of comets—Theory of Brédikhine—Spectra of comets—Comets of 1880 and 1882—The capture theory—Cometary photography CHAPTER VIII. 138 METEORS. Meteoric shower of 1833—Work of Olmsted—Work of Erman and Kirkwood—Of H. A. Newton—Adams and the meteoric orbit—Shower of 1866—Connection of comets and meteors—Work of Schiaparelli—Shower of meteors in 1872—‘Le Stelle Cadenti’—Meteoric observation—A. S. Herschel—Work of Denning—Stationary radiants—Bolides and aerolites—Origin of aerolites CHAPTER IX. 150 THE STARS. Distance of the stars—Life and work of Bessel—Studies of Struve—Life and work of Henderson—Work of Peters, Otto Struve, Brünnow, and Ball—Measures of Gill—Parallax of first-magnitude stars—Relative and absolute parallax—Work of Kapteyn—Application of photography—Star-catalogues—Argelander’s ‘Durchmusterung’—Work of Schönfeld—Work of Gould—The ‘Cape Photographic Durchmusterung’—Work of Gill and Kapteyn—International chart of the heavens—Work of Peck—Proper motions of the stars—Star-drift—Discoveries of Proctor and Flammarion—Radial motion—Work of Huggins, Vogel, and Campbell—Solar motion CHAPTER X. 169 THE LIGHT OF THE STARS. Work of Fraunhofer and Donati—Life and work of Secchi—His types of spectra—Life and work of Huggins—Photography of spectra—Life and work of Vogel—His classification of spectra—Work of Dunér—Of Pickering—Spectroscopic catalogues—Analysis of spectra—Stellar photometry—Life and work of E. C. Pickering—Variable stars—Work of Goodricke—Of Argelander, Schmidt, Heis, Schönfeld—Studies of Dunér—Of Gore—Photographic discoveries—Classification—Algol variables: their explanation—Explanation of other variables—η Argus—Temporary stars—Of 1848—Of 1866—Of 1876—Of 1885—Of 1892—Photographic discoveries—Nova Persei, 1901—New star of 1903—Theories of temporary stars CHAPTER XI. 197 STELLAR SYSTEMS AND NEBULÆ. Life and work of John Herschel—Binary stars—Computation of orbits—Work of Wilhelm Struve—Of Otto Struve—Of Burnham—Satellites of Sirius and Procyon—Astronomy of the invisible—Work of Pickering and Vogel—Spectroscopic binaries—Work of Bélopolsky and Campbell—Star-clusters—Nature of nebulæ—Spectroscopic work of Huggins—Of Copeland—Nebular photography—Work of Roberts, Barnard, Wolf—Of Keeler CHAPTER XII. 214 STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE. Work of John Herschel—Researches of Wilhelm Struve—Extinction of light—Mädler’s “central sun”—Distribution of nebulæ—Work of Proctor—Aggregation of stars on the Galaxy—Work of Gore and Schiaparelli—Studies of Gould—Researches of Kapteyn—Of Newcomb—Is the Universe limited? Newcomb’s argument—Observations of Celoria—Researches of Seeliger—External Universes—Gore’s speculations CHAPTER XIII. 227 CELESTIAL EVOLUTION. Laplace’s nebular hypothesis—Helmholtz and solar contraction—Theories of solar heat—Objections to Laplace’s theory—Faye’s hypothesis—Ball’s exposition—The meteoritic theory of Proctor—Its extension by Lockyer—Evolution of the stars—Vogel’s order of evolution—Tidal friction: work of Darwin—See’s explanation of double stars—Future of the Universe

A CENTURY’S PROGRESS IN ASTRONOMY.

CHAPTER I. HERSCHEL THE PIONEER.

In astronomy, as in other sciences, the past hundred years has been a period of unparalleled progress. New methods have been devised, fresh discoveries have been made, new theories have been propounded; the field of work has widened enormously. In fact, the science of the heavens has become not only boundless in its possibilities, but more awe-inspiring and marvellous.

To whom in the main is this great advance due? To the great pioneer of what may be called modern astronomy—William Herschel. Not only did Herschel reconstruct the science and widen its bounds, but his powerful genius directed the course of nineteenth century research. As an astronomical observer he has never been surpassed. In the breadth of his views he was equalled only by Newton; and indeed he excelled Newton in his unwearied observations and his sweeping conceptions of the Universe. To quote his own remark to the poet Campbell, he “looked farther into space than ever human being did before him.”

Herschel studied astronomy in all its aspects. In all the branches of modern astronomy he was a pioneer. He observed the Sun, Moon, and planets, devoting special attention to Mars and Saturn. He doubled the diameter of the Solar System by the discovery of Uranus. He discovered several satellites and studied comets. He was pre-eminently the founder of sidereal astronomy. He discovered binary stars, thus tracing the law of gravitation in the distant star-depths; while to him is due the credit of the discovery of the motion of the Solar System. He founded the study of star-clusters and nebulæ, propounded the nebular hypothesis, and devised two methods of star-gauging. Above all, he was the first to attempt the solution of one of the noblest problems ever attacked by man—the structure of the Universe. In fact, the latter problem was the end and aim of his observations. As Miss Clerke remarks, “The magnificence of the idea, which was rooted in his mind from the start, places him apart from and above all preceding observers.” Most of the departments of modern astronomy find a meeting-place in Herschel, as the branches run to the root of the tree. He discussed astronomy from every point of view. Before, however, proceeding to examine the work of this great man, it is well to note a few of his characteristics. These characteristics, once understood, give us the key to his researches. Before we can master Herschel the astronomer we must understand Herschel the man.

Notwithstanding the fact that Herschel spent most of his life in England, and that he is included in the ‘Dictionary of National Biography,’ he was pre-eminently a German. Like most Germans his style of writing was somewhat obscure, and this was emphasised when he wrote in English, owing to his imperfect command of the language. Had he written in German as well as in English, he would probably have been better understood in his native country, where erroneous views of his theories were long entertained. Even so distinguished an astronomer as Wilhelm Struve, when translating Herschel’s papers into German, made a mistake when translating a certain passage, which leaves the erroneous impression that Herschel believed the Universe to be infinite—a mistake which would not have arisen had he written in German.

The student of Herschel should also be careful in quoting the views of the great astronomer. Had Herschel at the close of his life written a volume containing his final views on the construction of the heavens, this would not have been necessary; but Herschel did not write such a volume. His researches were embodied in a series of papers communicated to the Royal Society from 1780 to 1818. As he observed the heavens his opinions progressed, so that a statement of his views at any given time was by no means a statement of his final opinions. The late R. A. Proctor, who was the first great exponent of Herschel in England, has well said: “It seems to have been supposed that his papers could be treated as we might treat such a work as Sir J. Herschel’s ‘Outlines of Astronomy’; that extracts might be made from any part of any paper without reference to the position which the paper chanced to occupy in the entire series.”

Herschel, like the true student of nature, held theories very lightly. They were to him but roads to the truth. Unlike many scientists, he did not interpret observations by hypothesis: he framed his theories to fit his observations. If he found that a certain theory did not agree with what he actually saw in the heavens, he abandoned it: he did not hesitate to change his views as his investigations proceeded. “No fear of ‘committing himself,’” says Miss Clerke in her admirable work on ‘The Herschels,’ “deterred him from imparting the thoughts that accompanied his multitudinous observations. He felt committed to nothing but truth.”