Sir William Herschel

views updated Jun 08 2018

Sir William Herschel

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 Nov. 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 Aug. 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).

Additional Sources

Armitage, A. (Angus), William Herschel, London, New York, Nelson 1962.

Clerke, Agnes M. (Agnes Mary), 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. (Constance Ann), 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. □

Herschel, Sir William (1738-1822)

views updated May 11 2018

Herschel, Sir William (1738-1822)

German-born English astronomer

Sir William Herschel was among the preeminent astronomers of the eighteenth century, and is credited with discovering the planet Uranus, binary stars, nebulae, and for correctly describing the form of the Milky Way galaxy.

Herschel was born in Hanover, Germany at a time when the city belonged to England under the rule of George II. As Herschel's father was a musician in the Hanoverian army, Herschel himself was trained in music in order to enter the same profession. The Seven Years' War, however, made military life an unattractive option, and in 1757, Herschel arrived in England where he began working as an organist and music teacher. Herschel learned of astronomy through his interest in the theory of music and the scientific basis for musical sounds, which led him to mathematics and then optics.

Newton's treatise on optics inspired Herschel with his desire to study the stars. Unable to find a telescope of a high enough resolution, he decided to grind his own lenses and to design his own instruments. With his first telescope, a 6-foot Gregorian reflector that was one of the best of its kind, he decided that its first application would be to conduct a systematic survey of the stars and planets. Herschel was assisted in this endeavor by his sister Caroline, who also discovered eight comets and produced two astronomy catalogues in her lifetime. Throughout his life, Herschel built numerous telescopes, each one more sophisticated and more powerful than the last.

Herschel's first major discovery occurred in 1781, during his second survey of the sky when he announced the existence of a new planet to be found in the constellation of Taurus. Herschel's name for the new planet was Georgium Sidus, George's star, in honor of King George III, but it eventually came to be known as Uranus, after the mythical father of Saturn. The discovery of Uranus, which effectively doubled the previously accepted size of the solar system , caused a popular and scientific sensation, and George III appointed Herschel to the position of King's Astronomer while providing him with a small annuity that allowed him to pursue astronomy full time.

Herschel's most significant achievements were in the area of sidereal astronomy, to which he contributed the first systematic body of evidence on the order and nature of the stars and the planets. Whereas plenty of theories had been put forward by prominent philosophers of the time on the systems that might govern the universe, none were supported by any scientific gathering of data. In 1783, Herschel began to search for nebulae in the sky, and raised their known total from little more than 100 to 2,500. Much of eighteenth-century astronomy set out to determine the distances between stars; trigonometrical calculations based on their apparent annual movement, however, had failed. Galileo Galilei had proposed the use of double stars, pairs of stars very close together, to calculate stellar distance, where the fainter member of the pair was so far away as to represent a fixed point from which the annual movement of its brighter companion could be measured. In Herschel's second survey, he searched for double stars, producing three catalogs over the next 40 years and listing 848 examples. It was later discovered by another astronomer who had seen Herschel's work that these double stars were in fact companions in space held together by gravitational forces and therefore equidistant from the earth; Herschel had assumed that companions in space would have been of equal brightness, and had therefore discounted this possibility. Nonetheless, much of Herschel's work was concerned with producing evidence for the powers of attraction between stars. In three of his papers delivered between 1784 and 1789, he proposed a cosmogony for the universe in which stars, initially randomly scattered throughout the universe, clustered together over time around the regions from which they originally developed.

Herschel was the first to embark upon a scientific study of the Milky Way and half of his work, though less influential, focuses upon the solar system. He studied the Sun , observing that what we see is not the Sun itself but the clouds of gases that cover its surface, and examined the nature of the infrared section of the spectrum by which some of the Sun's heat is transmitted. Besides calculating the height of lunar mountains, Herschel devoted most of his attention to the other known planets, Venus, Mars, Jupiter, and Saturn, determining their rotation period and checking the inclination of their axes, their shape, and the nature of their atmospheres. Herschel devoted most of his attention to examining Saturn and its rings, arguing at one point that the rings were solid, but later conceding that they were in fact composed of floating particles.

Herschel's work on nebulae had led him to conclude that they might well be other solar systems seen only as a luminous cluster of stars around a brighter one. As a result, he saw the Milky Way and Earth as only one rather insignificant part of the universe. In this sense, he changed the status of the solar system within the universe in much the same way as Nicolas Copernicus had the earth when he showed that the planets revolved around the Sun rather than Earth.

See also Cosmology

Herschel, Sir William

views updated Jun 11 2018

Herschel, Sir William (1738–1822) English astronomer, b. Germany. He discovered Uranus (1781) and later two satellites of Uranus (1787) and two of Saturn (1789). Herschel observed many double stars and more than 2000 nebulae and clusters, and published catalogues of them. Herschel realized that the Milky Way is the plane of a disc-shaped universe, whose form he calculated by counting the numbers of stars visible in different directions. In 1800, he discovered and investigated infrared radiation. See also nebula