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Summary Article: Herschel, (Frederick) William (1738-1822)
From The Hutchinson Dictionary of Scientific Biography

Place: Germany

Subject: biography, astronomy

German-born English astronomer who contributed immensely to contemporary scientific knowledge. Through determined efforts to improve the quality of his telescopes, he was able to use the finest equipment of his time, which in turn permitted him to make many significant discoveries about the nature and distribution of stars and other bodies, both within the Solar System and beyond it. He was the most influential astronomer of his day.

Herschel was born in Hanover, Germany, on 15 November 1738. At the age of 14 he joined the regimental band of the Hanoverian Guards as an oboist (as his father had before him), and four years later he visited England with the band. In 1757 he emigrated to the UK, going first to Leeds - where he earned his living copying music and teaching - and then, three years later, by commission from the Earl of Darlington, to Durham to become conductor of a military band there. From 1761 to 1765 Herschel worked as a teacher, organist, composer, and conductor in Doncaster. He then did similar work in Halifax for a year. In 1766 he was hired as organist at the Octagon Chapel in Bath, where he remained for the next 16 years; during this time he ran a tutorial service that helped to finance his growing interest in astronomy. In 1772 he went to Hanover to bring his sister Caroline Herschel back to England. She too became fascinated by astronomy and helped Herschel enormously both in the delicate task of preparing instruments and in making observations. The serious astronomical work began in 1773, with the building of telescopes and the grinding of mirrors. Herschel's first large reflector was set up behind his house in 1775.

Herschel's discovery of the planet Uranus in 1781 - the first planet to have been discovered in modern times - created a sensation: it signalled that Isaac Newton's work had not covered everything there was to know about the universe. Herschel originally named the new planet ‘Georgium Sidum’ (George's Star) in honour of King George III, but the name Uranus - proposed by Johann Bode of the Berlin Observatory - was ultimately accepted. Nevertheless, the king was flattered, and Herschel received a royal summons to bring his equipment to court for inspection. In the same year the Royal Society elected Herschel a fellow, and awarded him the Copley Medal. In 1782 he was appointed court astronomer, a post that carried with it a pension of 300 guineas per year. This enabled him to give up teaching and to move from Bath to Windsor, and then to Slough, although he continued to make telescopes for sale in order to supplement his income until 1788 (when he married a wealthy widow). In addition, the king provided grants for the construction of larger instruments; for instance, he provided £4,000 for a telescope with a focal length of more than 12 m/40 ft and a reflector that was 1 m/3 ft in diameter. (This telescope proved rather cumbersome and was not used after 1811, although for many years it remained the largest in the world.)

Herschel visited Paris in 1801, meeting Pierre Laplace and Napoleon Bonaparte. Many honours were conferred upon him (he was knighted in 1816), and he was a member of several important scientific organizations. He became ill in 1808, but continued to make observations until 1819. In that year his only son, John Herschel, finished his studies at Cambridge and came to take over his father's work. Herschel died in Slough on 25 August 1822.

Herschel's first large telescope was a 1.8-m/6-ft Gregorian reflector that he built himself in 1774. In many ways it was better than other existing telescopes, and Herschel decided that its primary use would be to make a systematic survey of the whole sky. His first review was completed in 1779, when he immediately began a second survey with a 2.1-m/7-ft reflector. In this project he concentrated on noting the positions of double stars, that is, stars that appear to be very close together, perhaps only as a consequence of chance alignment with the observer. His first catalogue of double stars was published in 1782 (with 269 examples); a second catalogue (with 434 examples) appeared in 1785; and a third was issued in 1821, bringing the total number of double stars recorded to 848. Galileo and others had suggested that the motion of the nearer (and it was presumed therefore brighter) star relative to the more distant (thus fainter) star could be used to measure annual movements of the stars. Herschel's observations in 1793 demonstrated that a correlation between dimness and distance did not apply in all cases, and that in fact some double stars were so close together as to rotate round each other, held together by an attractive force. This was the first indication of gravity acting on bodies outside the Solar System.

Herschel's work on bodies within the Solar System included an accurate determination of the rotation period of Mars (in 1781): 24 h, 39 min, 21.67 sec - only two minutes longer than the period now accepted. By inventing an improved viewing apparatus for his telescope, particularly valuable when little light was available, in 1787 he discovered Titania and Oberon, two satellites of Uranus. Incorporating the ‘Herschelian arrangement’ into a massive telescope 12 m/39 ft long, Herschel then discovered two further satellites of Saturn (Enceladus and Mimas) on its first night of use. Saturn continued to be an object of great interest to him.

During the 1780s Herschel published a number of papers on the evolution of the universe from a hypothetical uniform initial state to one in which stars were clumped into galaxies (seen as nebulae). Herschel had become interested in nebulae in 1781, when he was given a list of a hundred of these indistinct celestial bodies compiled by Charles Messier. Herschel began looking for more nebulae in 1783, and the improved resolving power of his telescopes enabled him to see them as clusters of stars. His first catalogue of nebulae (citing no fewer than 2,500 examples) was published in 1802; an even longer catalogue (with 5,000 nebulae) appeared in 1820.

In 1800 Herschel examined the solar spectrum using prisms and temperature-measuring equipment. He found that there were temperature differences between the various regions of the spectrum, but that the hottest radiation was not within the visible range, but in the region now known as the infrared. This was the beginning of the science of stellar photometry. Using data published by Nevil Maskelyne on seven particularly bright stars, Herschel demonstrated that if the Sun's motion towards Argelander (a star in the constellation Hercules) was accepted, then the ‘proper motion’ of the seven stars was a reflection of the motion of the observer. This relegation of the Solar System from the centre of the universe was, in its way, analogous to the dethronement of the Earth from the centre of the Solar System by Copernicus. Herschel also measured the velocity of the Sun's motion.

An industrious and dedicated astronomer, and a very practical man, Herschel contributed enormously to the advance of scientific progress.

© RM, 2018. All rights reserved.

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