Herschel ,.I. Sir William, an English astronomer, born in Hanover, Nov. 15, 1738, died at Slough, near Windsor, Aug. 23, 1822. His father, a musician, educated him to his own profession, and at the age of 14 placed him in the band of the Hanoverian foot guards. In 1757 he went to England to seek his fortune, and for some years he devoted himself to music for support. He is said to have been engaged in military bands and at concerts, but there is much confusion in the stories relating to this period of his life. He became organist at Halifax, and in 1700 at the Octagon chapel in Bath. In the latter place he first turned his attention to the study of astronomy, particularly to the construction of optical instruments. In 1774 he had made a large reflecting telescope. While at Bath he constructed 200 Newtonian telescopes of 7 ft. focus, 150 of 10 ft, and about 80 of 20 ft., and did far more than any one who had preceded him in uniting to the best advantage the magnifying and the illuminating power of the telescope. Either of these qualities may easily be strengthened, but at the expense of the other, and the exact proportion in which they must be united to render the greatest amount of light effective was a problem which required many careful experiments.
With one telescope, magnifying 227 times, Herschel began a careful survey of all the stars, serially; and while examining the constellation of Gemini, he noticed (March 13, 1781) that one of them appeared unusually large, and a second examination showed it to have changed its place. Finally he pronounced it a comet, and it was so published in the "Philosophical Transactions" (1781). This announcement drew the attention of astronomers to the supposed comet, and they began to endeavor to compute its course. The president Saron first pronounced it a planet, and then Lexell and Laplace, almost simultaneously, computed its elements, and found it to have an elliptical orbit, whose great axis was about 19 times greater than that of the earth, and the period of its revolution to be 84 years. Herschel had taken no part in the mathematical calculations, but on its being pronounced a planet, he proposed to name it the Georgium Sidus. It has often been called Herschel, but the name Uranus, applied to it by Bode, has been generally adopted.
Herschel now turned his attention most carefully to this planet, determined the apparent diameter (about 4") for its mean distance from the earth, and discovered two of its satellites, revolving in a plane nearly perpendicular (at an angle of 78° 58') to its orbit, and contrary to the order of signs (that is, from east to west). He thought he had also detected four other satellites; but it is now generally believed that he mistook faint stars for satellites, and that Uranus has only four, two of which were discovered by Lassell, of England, in 1851. The discovery of Uranus attracted the attention of all Europe, and Herschel was made private astronomer to the king, with a salary of £400 and a house near Windsor, first at Datchet, and finally at Slough. With funds advanced by the king, Herschel constructed his celebrated 40-foot reflecting telescope, the metal speculum of which was 4 ft. in diameter, 3 1/2 in. thick, and over 2,000 lbs. in weight. The plane mirror of the instrument was dispensed with, and the observer sat in a swinging chair with his back to the object observed, and facing the object end of the tube, in which the image, by an inclination of the speculum, was thrown to one side and observed through a single lens.
He conjectured that with this instrument 18,000,000 stars might be seen in the milky way. - Though Herschel discovered an almost unprecedented number of new bodies in the planetary system, yet his glory is greatest in sidereal astronomy, of which he laid almost the foundations. His leading discoveries in this branch of the science were the following: I. The binary system of stars, and the orbits of several revolving stars. Double stars had been noticed even before the introduction of the telescope; but while Herschel was observing them to learn their annual parallax, he noticed a steadily progressive change in their position and distance; and in 1802, 23 years after he began his observations, he announced in the "Philosophical Transactions" his discovery that both stars were revolving round their common centre of gravity, and all his instances have been confirmed. II. He classified the nebulae, and advocated the nebular hypothesis, since supposed to be disproved by the discoveries made with the great telescope of Lord Rosse, but now accepted as demonstrated by the results of spectroscopic analysis. He discovered that these nebulous spots cover at least 1/270 of the visible firmament, and in 1802 he indicated the positions of 2,500 nebula) or clusters of stars.
He classified them as: 1, clusters of stars; 2, nebula) proper; 3, nebulous stars. III. The law of grouping the entire visible firmament. He "gauged" the heavens, by counting the whole number of stars visible in the field of his 20-foot reflector, and taking the average for each region. The result showed a remarkable and steady law of decrease, from the central zone of the milky way in opposite directions to the northern and southern poles. IV. The determination of the fact of the motion of our system, and the di-rection of that motion. It was already known that the stars were not fixed, but had a proper motion. Herschel, from the proper motions of about 20 stars, with great penetration, divined that our system was moving in the direction of Herculis, a point whose right ascension is 270°, and north declination 25°. Besides discovering the satellites of his own planet, Herschel discovered two new satellites of Saturn, now called, from their being next the ring, the first and second, and determined the rotation of the rings of the planet to be in 10 h. 32 m. He found also that the time of the rotation of the satellites of Jupiter was just equal to the period of their revolution about the planet. When his age made it advisable for him to discontinue his observations in the heavens, he turned his attention to the properties of heat and light. He also gave some valuable opinions concerning the spots on the sun, attributing them to occasional openings in the luminous coating, which seems to be always in motion. - Herschel contributed papers, sometimes several in a year, to the "Philosophical Transactions" from 1780 to 1815. He married in 1788 Mrs. Mary Pitt, a widow of considerable fortune, and had by her one son, John. II. Caroline Lucretia, sister of the preceding, born in Hanover, March 16, 1750, died there, Jan. 9, 1848. She lived in Hanover till her 22d year, when she went to England to join her brother at Hath. Here she turned her attention to astronomy, and gave him great assistance, not only taking the part of an amanuensis, but frequently performing alone the long and complicated calculations involved in the observations.
For this she received a pension from George III. Meanwhile she took her own separate observations of the heavens, with a small Newtonian telescope which her brother had made for her. She devoted herself particularly to a search for comets, and between 1786 and 1805 discovered alone eight of these bodies, of five of which she was the first observer. Her contributions to science, most of them in her brother's works and under his name, are very valuable. She took the original observations of several remarkable nebulae in her brother's catalogue, and computed the places of his 2,500 nebula). In 1798 she published her "Catalogue of Stars taken from Mr. Flamsteed's Observations, contained in the second volume of the Historia Coelestis, and not inserted in the British Catalogue, with an index to point out every observation in that volume belonging to the stars of the British Catalogue; to which is added a collection of Errata that should be noticed in the same volume." This work was published at the expense of the royal society, and contained about 560 stars which had been omitted by the framers of the British catalogue. After her brother's death she returned to her native city.
In 1828 she completed a catalogue of the nebulae and stars observed by her brother, for which she received a gold medal from the astronomical society of London, and was elected an honorary member of it. III. Sir John Frederick William, an English astronomer and physicist, son of Sir William Herschel, born at Slough, March 7, 1792, died at Collingwood, near Hawkhurst, May 11, 1871. At Cambridge, where he graduated in 1813, he was distinguished for his mathematical genius and his fondness for physical science. In 1820 he published his "Collection of Examples of the Application of the Calculus to Finite Differences." About 1825 he began his observations in sidereal astronomy, to which he chiefly devoted himself, partly in conjunction with Sir James South, and the results of his observations for eight years were communicated to the royal astronomical society in a series of catalogues, the first appearing in 1825, for which he received the gold medal. In 1830 he published important measurements of 1,236 stars, which he found with his 20-foot reflector.
In 1830 he wrote for the "Encyclopaedia Metropolitana" a treatise on "Sound," and for the same work in 1831 a treatise on the "Theory of Light." In Lardner's "Cyclopaedia" he published a " Preliminary Discourse on the Study of Natural Philosophy," and a "Treatise on Astronomy." About the same time he wrote several experimental essays on different branches of chemistry, magnetism, and optics. His great enterprise was his expedition to the Cape of Good Hope, to take observations of the whole firmament of the southern hemisphere. Taking with him the same instruments (a 20-foot reflector with an 18 1/4 inch aperture, and a 7-foot achromatic with a 5-inch aperture) which he had used in the northern hemisphere, that his results might be compared with his former ones, he arrived at the Cape, Jan. 15, 1834, and settled at Feldhuysen, about 6 m. from Table bay. He examined carefully and measured the double stars, clusters, and nebulae of the southern skies, and completed the wonderful " gauging of the heavens " which had been begun by his father. His observations lasted four years, and the entire expense was defrayed by himself, though an ample indemnity was offered him by government.
During his absence, in 1836, the royal astronomical society again voted him their gold medal, and on his return honors were heaped upon him. The royal society proposed to make him their president, but he was unwilling to accept the office. In 1838, at the coronation of Queen Victoria, he was created a baronet. In 1839 he received the degree of D. C. L. from Oxford, and in 1842 he was elected lord rector of Marischal college, Aberdeen. In 1847 appeared in a large 4to volume his "Results of Astronomical Observations made during the Years 1834-'8 at the Cape of Good Hope, being the completion of a Telescopic Survey of the whole surface of the Visible Heavens, commenced in 1825." This work, one of the most considerable and valuable of our time, is divided into seven portions: 1, "Nebulae of the Southern Hemisphere;". 2, " The Double Stars of the Southern Hemisphere;" 3, "Astrometry, or the Numerical Expression of the Apparent Magnitudes of Stars;" 4, "The Distribution of Stars, and the Constitution of the Galaxy in the Southern Hemisphere;" 5, "Observations of Halley's Comet (as seen at the Cape toward the close of 1835), with Remarks on its Physical Condition and that of Comets in general;" 6, "Observations of the Satellites of Saturn;" 7, "Observations of Solar Spots." His residence at the Cape gave not only valuable additions to astronomy, but also to meteorology.
He suggested the plan of taking simultaneous meteorological observations at different places on given days, and embodied his views on the plan in his " Instructions for Making and Registering Meteorological Observations at various Stations in Southern Africa " (1844). Before going to the Cape of Good Hope he added 800 nebulae to the catalogue of his father, and on his return published a catalogue of 2,049 nebulae of the southern hemisphere and their positions, 500 of which were before entirely unknown. He also added, while at the Cape, 1,081 double stars, and in measuring the angles of positions and the distances of the stars from each other, found that many of them have very rapid orbital motions. He made many interesting observations on the milky way. "This remarkable belt," he says, "examined through a powerful telescope, is found (wonderful to relate) to consist entirely of stars scattered by millions, like glittering dust, on the black ground of the general heavens." Again, he conjectures, from his ingenious combinations of photometric calculations, that if the stars in the great circle of the milky way, which he saw in his 20-foot reflecting telescope, were newly risen luminous cosmical bodies, it would require 2,000 years for a ray of their light to reach us.
His observations on the brightness and the color of stars, on variable stars, on the sun's rays, on the atmospheric air, and on the Magellanic clouds, are all very valuable. Sir John Herschel did not confine his attention to astronomy. He calculated the density of the atmosphere, and held that a perfect vacuum exists at the height of 80 or 90 miles above the earth, and also that three fourths of all the atmospheric air is within four miles of the earth's surface. The question concerning the absorption of light, which gave rise to much discussion, particularly in its connection with the undulatory theory, was very ably answered by Herschel in his paper on the "Absorption of Light by Colored Media." He made some important discoveries in photography, and produced from chemical compounds and the juices of plants the most beautiful chromatic effects. Sir John Herschel contributed to the " Manual of Scientific Inquiry " (1849 and 1851), and wrote "Outlines of Astronomy" (1850; 10th ed., 1869); " Essays, from the Edinburgh and Quarterly Reviews, with Addresses and other Pieces" (1857); "Physical Geography "(1861); and "Familiar Letters on Scientific Subjects " (18GG). His son, Capt. John Herschel of the royal engineers, is now (1874) collecting his letters with a view to publication.
During the year 1848 Herschel was president of the royal astronomical society. In 1850 he was appointed master of the mint, which office he held till 1855, when he resigned on account of ill health. In 1855 he became one of eight foreign associates of the French academy of science.