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display had been witnessed in Mexico by Humboldt and Bonpland on November 12th, 1799. H. A. Newton traced such records back to October 13th, A.d. 902. The orbital motion of a cloud or stream of small particles was indicated. The period favoured by H. A. Newton was 354J days; another suggestion was 375J days, and another 33J years. He noticed that the advance of the date of the shower between 902 and 1833, at the rate of one day in seventy years, meant a progression of the node of the orbit. Adams undertook to calculate what the amount would be on all the five suppositions that had been made about the period. After a laboriouswork, he found that none gave one day in seventy years except the 33^-year period, which did so exactly. H. A. Newton predicted a return of the shower on the night of November i3th-i4th, 1866. He is now dead; but many of us are alive to recall the wonder and enthusiasm with which we saw this prediction being fulfilled by the grandest display of meteors ever seen by anyone now alive.
The progression of the nodes proved the path of the meteor stream to be retrograde. The radiant had almost the exact longitude of the point towards which the earth was moving. This proved that the meteor cluster was at perihelion. The period being known, the eccentricity of the orbit was obtainable, also the orbital velocity of the meteors in perihelion; and, by comparing this with the earth's velocity, the latitude of the radiant enabled the inclination to be determined, while the longitude of the earth that night was the longitude of the node. In such a way Schiaparelli was able to find first the elements of the orbit of the August meteor shower (Perseids), and to show its identity with the orbit of Tuttle's comet 1862.iii. Then, in January, 1867, Le Verrier gave the elements of the November meteor shower (Leonids); and Peters, of Altona, identified these with Oppolzer's elements for Tempel's comet 1866 — Schiaparelli having independently attained both of these results. Subsequently Weiss, of Vienna, identified the meteor shower of April 20th (Lyrids) with comet 1861. Finally, that indefatigable worker on meteors, A. S. Herschel, added to the number, and in 1878 gave a list of seventy-six coincidences between cometary and meteoric orbits.
Cometary astronomy is now largely indebted to photography, not merely for accurate delineations of shape, but actually for the discovery of most of them. The art has also been applied to the observation of comets at distances from their perihelia so great as to prevent their visual observation. Thus has Wolf, of Heidelberg, found upon old plates the position of comet 1905.v., as a star of the 15.5 magnitude, 783 days before the date of its discovery. From the point of view of the importance of finding out the divergence of a cometary orbit from a parabola, its period, and its aphelion distance, this increase of range attains the very highest value.
The present Astronomer Royal, appreciating this possibility, has been searching by photography for Halley's comet since November, 1907, although its perihelion passage will not take place until April, 1910.
15. The Fixed Stars And Nebula
Passing now from our solar system, which appears to be subject to the action of the same forces as those we experience on our globe, there remains an innumerable host of fixed stars, nebulae, and nebulous clusters of stars. To these the attention of astronomers has been more earnestly directed since telescopes have been so much enlarged. Photography also has enabled a vast amount of work to be covered in a comparatively short period, and the spectroscope has given them the means, not only of studying the chemistry of the heavens, but also of detecting any motion in the line of sight from less than a mile a second and upwards in any star, however distant, provided it be bright enough.
In the field of telescopic discovery beyond our solar system there is no one who has enlarged our knowledge so much as Sir William Herschel, to whom we owe the greatest discovery in
Sir William Herschel, F.R.S.—1738-1822.
Painted by Lemuel F. Abbott; National Portrait Gallery, Room XX.
dynamical astronomy among the stars — viz., that the law of gravitation extends to the most distant stars, and that many of them describe elliptic orbits about each other. W. Herschel was born at Hanover in 1738, came to England in 1758 as a trained musician, and died in 1822. He studied science when he could, and hired a telescope, until he learnt to make his own specula and telescopes. He made 430 parabolic specula in twenty-one years. He discovered 2,500 nebulas and 806 double stars, counted the stars in 3,400 gauge-fields, and compared the principal stars photometrically.
Some of the things for which he is best known were results of those accidents that happen only to the indefatigable enthusiast. Such was the discovery of Uranus, which led to funds being provided for constructing his 40-foot telescope, after which, in 1786, he settled at Slough. In the same way, while trying to detect the annual parallax of the stars, he failed in that quest, but discovered binary systems of stars revolving in ellipses round each other; just as Bradley's attack on stellar parallax failed, but led to the discovery of aberration, nutation, and the true velocity of light.
Parallax. — The absence of stellar parallax was the great objection to any theory of the earth's motion prior to Kepler's time. It is true that Kepler's theory itself could have been geometrically expressed equally well with the earth or any other point fixed. But in Kepler's case the obviously implied physical theory of the planetary motions, even before Newton explained the simplicity of conception involved,