The German-born English astronomer Sir William Herschel (1738-1822) discovered the planet Uranus, the intrinsic motion of the sun in space, and the form of the Milky Way.

William (originally Friedrich Wilhelm) Herschel was born in Hanover on November 15, 1738. His father was a musician in the Hanoverian guard, which William joined at the age of 14.

In 1757 Herschel went to England. In Yorkshire he conducted a small military band, and from 1762 to 1766 he was a concert manager in Leeds. His notebook of 1766 has these laconic entries: "Feb. 19. Wheatly. Observation of Venus" and "Feb. 24. Eclipse of the moon at 7 o'clock A.M. Kirby." These are the first signs of Herschel's future interests. By the end of the year he became organist at the fashionable spa town of Bath. In 1772 his sister, Caroline Lucretia Herschel, came to live with him at Bath. She collaborated with her brother on astronomical researches.

Not until 1773 is there another scientific entry in Herschel's notebooks: "April 19. Bought a quadrant and Emerson's Trigonometry." That this entry heralded a new phase in his life is shown by the fact that it is followed by others of a similar nature: "Bought a book of astronomy ... bought an object glass ... bought many eye glasses ... hire of a 2 feet reflecting telescope." These entries show that he was proposing to make his first (metal) telescope mirror.

Herschel's First Telescope

Obsessed with astronomy, Herschel progressed through pasteboard and tin-tubed telescopes to a hired Gregorian reflector. When he tried to buy a much larger reflecting telescope in London, he could find nothing suitable which he could afford. For this reason he began to build his own. By September 1774 he was observing the heavens with a (Newtonian) reflecting telescope of 6-foot focal length of his own construction.

Herschel now entered into a long and tedious period of his life, when he and his brother and sister worked away at grinding and polishing telescope mirrors. He had to keep the mirror moving unceasingly on the grinding tool for long periods of time. His sister fed him as he worked. Some idea of his astonishing industry may be had from his statement, made in 1795, that he had made "not less than 200 7 feet, 150 10 feet and about 80 20 feet mirrors." Of the various mountings he devised for these, he was very pleased with a 7-foot Newtonian telescope stand, completed in 1778.

Early Observations

Herschel began to keep a record of what he saw in the heavens from March 1, 1774. He observed the rings of Saturn, the moons of Jupiter, and the markings of the moon. It is interesting to see how in his eagerness to make novel discoveries he was deluded into thinking that he had found signs of a forest on the moon, even supposing that he could make out the shadows cast by the trees at the edge of the wood. His next lunar observations were 3 years later, when he began to calculate the height of the lunar mountains.

This self-taught astronomer of Bath was by his own efforts soon to be transmuted into the world's leading observational astronomer. He possessed instruments as powerful as any to be found and all the perseverance needed to use them effectively. In 1777 he began observations of a well-known but neglected star, Mira Ceti, which varies in brightness periodically. Soon he had the idea of determining the annual parallax of stars (the shift in the apparent relative positions of the stars as the earth goes around the sun). Whether the stars were so far away as to make this apparent movement unobservable was not then known. In fact, no annual parallax was measured until 1838, when Friedrich Wilhelm Bessel measured that of star 61 Cygni. Herschel, nevertheless, observed the relative positions of pairs of stars close together (called double stars). He measured hundreds of double stars, but in March 1778 he recorded his disappointment at finding "the stars in the tail of Ursa Major just as I saw them three months ago, at least not visibly different."

Discovery of Uranus

In recording double stars systematically, on March 13, 1781, Herschel entered a pair of which "the lowest of the two is a curious either nebulous star or perhaps a comet." Four days later he looked for the object and found that it had moved. He recorded the new position of the "comet" and proceeded to follow it regularly. What he had discovered was the planet Uranus, as it is now known--the first planet to be discovered in historical times.

Herschel was given the Copley Medal of the Royal Society and elected a fellow. Col. John Walsh wrote to him that he had spoken with the king, George III, and had taken "occasion to mention that you had a twofold claim as a Native of Hanover and a Resident of Great Britain, where the Discovery was made, to be permitted to name the Planet from his Majesty." The planet was thus at first called "Georgium sidus" ("star George"), and it appears in this form on early maps and models.

King's Patronage

George III asked Herschel to move his telescope to an observatory the King had built in the Deer Park at Richmond. Herschel moved to Windsor, near the King's residence, and in due course was given the patronage for which he had long hoped--a salary for himself and his sister, upkeep for the telescope, and later a very large sum for a 40-foot telescope, the largest ever made before the mid-19th century.

Herschel eventually settled at Slough, where he wrote the paper announcing his second great discovery, "Motion of the Solar System in Space" (1783). He carefully noted the proper motions of seven bright stars and showed that the movement in the intervening time seemed to converge on a fixed point, which he interpreted correctly as the point from which the sun is receding. Other discoveries followed. He found that "Georgium sidus" had satellites. Some of those he discovered are now known to be spurious, but the difficulties of observing, especially with the crude mounting available to him, were very great.

Structure of the Universe

Many double stars are seen as such merely because they happen to be in a straight line as seen from the earth. Herschel reasoned that if one member of a double-star system was much brighter than the other this must be the result of such a coincidence, the brighter star of the pair being much the closer of the two. He continued to record the relative positions of all such systems, and in 1782 and 1785 he presented long lists of his observations. He was, of course, assuming that the stars were all more or less uniformly bright, intrinsically speaking, and that they were uniformly distributed throughout space. This being so, he believed that by taking counts of stars over a given small area of sky the number of stars visible would give him the extent of the Milky Way in that direction. He thus formulated a picture or map of the Milky Way, which was quite remarkable in his time, and which even now is not wildly wrong.

Later Years

In 1788 Herschel married Mary Pitt, a wealthy widow, by whom he had his only son. Herschel was able to make a useful additional income by selling telescopes, and he invested money in building machines to help grind mirrors. He corresponded with the leading astronomers of England and Europe and received many distinguished visitors at Slough who were anxious to see the telescope he had completed on Aug. 28, 1789; it had a 40-foot focal length and 4-foot aperture.

Herschel was knighted in 1816 and received honors from states and academies the world over. He died at Slough on August 25, 1822.

Further Reading

• The most important volumes for an appreciation of Herschel are The Scientific Papers of Sir William Herschel (2 vols., 1912), the first volume of which has an invaluable short account of Herschel's life by J. L. E. Dreyer. Biographies include Edward S. Holden, Sir W. Herschel: His Life and Works (1881); James Sime, William Herschel and His Works (1900); and J.B. Sidgwick, William Herschel: Explorer of the Heavens (1953). Michael A. Hoskin, William Herschel and the Construction of the Heavens (1963), is composed largely of extracts from Herschel's writings, intended to show his views on the structure of the universe. For a short account of Herschel's views and the way in which they were developed by others see J. D. North, The Measure of the Universe: A History of Modern Cosmology (1965).
• Armitage, A., William Herschel, London, New York, Nelson 1962.
• Clerke, Agnes M., The Herschels and Modern Astronomy, London, New York etc. Cassell and Company, Limited, 1901.
• Crawford, Deborah, The King's Astronomer, William Herschel, New York, J. Messner 1968.
• Hoskin, Michael A., William Herschel and the Construction of the Heaven, New York, Norton 1964, 1963.
• Lubbock, Constance A., 1855?-1939, ed., The Herschel Chronicle; The Life-Story of William Herschel and His Sister, Caroline Herschel, New York, The Macmillan Company; Cambridge, Eng., The University Press, 1933.
• Moore, Patrick, William Herschel, Astronomer and Musician of 19 New King Street, Bath, Sidcup, Kent, England: P.M.E. Erwood in association with The William Herschel Society, Bath, England, 1981.