Michell, John
MICHELL, JOHN
(b. Nottinghamshire [?], England, 1724[?]; d. Thornhill, near Leeds, England, 21 April 1793)
astronomy.
Michell earned a permanent place in the history of stellar astronomy for two signal accomplishments: he was the first to make a realistic estimate of the distance to the stars, and he discovered the existence of physical double stars. He was educated at Cambridge. After graduating from Queens’ College with the M.A. (1752) and the B.D. (1761), he held the Woodwardian chair of geology at Cambridge (1762–1764). In 1767 he was appointed rector of St. Michael’s Church in Thornhill, near Leeds—a post he held for the rest of his life. He is buried at Thornhill, where the parish register describes him as aged sixty-eight (hence the surmise that he was born in 1724).
Michell’s published scientific work, which earned him election to the Royal Society in 1760, covered many subjects, including the cause of earthquakes (1760), observations of the comet of January 1760, a method for measuring degrees of longitude “upon parallels of the Equator” (1766), and an independent discovery with Coulomb of the torsion balance (1784). His greatest accomplishments were two investigations published in the Philosophical Transactions of the Royal Society: “An Inquiry Into the Probable Parallax and Magnitude of the Fixed Stars From the Quantity of Light Which They Afford Us, and the Particular Circumstances of Their Situation” (1767) and “On the Means of Discovering the Distance, Magnitude, etc. of the Fixed Stars” (1784).
In the first of these papers, Michell pointed out that the frequency of the angular separation of close pairs of stars known at that time deviated grossly from what one could expect for chance projection of stars uniformly distributed in space—there appeared to be an excessive number of close pairs—and, according to Michell: “… The natural conclusion from hence is, that it is highly probable, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed very near together, do really consist of stars placed nearly together, and under the influence of some generallaw … to whatever cause this may be owing, whether to their mutual gravitation, or to some other law or appointment of the Creator.” The directness of Michell’s language perhaps leaves something to be desired; but the unimpeachable logic of his arguments gave a convincing theoretical proof of the existence of physical binary stars in the sky long before Herschel (1803) provided a compelling observational proof.
Michell’s second great achievement was a realistic estimate of the distance to the stars, and he made it more than half a century before the first parallax of any fixed star had been measured. His argument was very neat and can be regarded as the precursor of the “photometric” parallaxes of the twentieth century. Michell noticed that Saturn at opposition appears in the sky as bright as the star Vega and exhibits an apparent disk about twenty seconds in diameter, one which from the sun would be seen as seventeen seconds across. Therefore Saturn’s illuminated hemisphere clearly intercepts (17/3600)2(│/720)2 of the light sent out by the sun.
Now—and this is essential—if the sun and Vega were of equal intrinsic brightness, and Vega’s apparent brightness is equal to that of Saturn, it follows (from the inverse-square law of the attenuation of brightness, already established by Bouguer) that Vega must be (360O/17)(72O/│), or 48,500, times as far from the sun as Saturn is. Moreover, since Saturn is known to be 9.5 times as far from the sun as the earth is, it follows that the distance to Vega should amount to 9.5 X 48,500, or some 460,000 astronomical units.
Although this value represents only about a quarter of the actual distance of Vega,first measured trigonometrically by K F. G. W. Struve in 1837 (the underestimate resulting from Vega’s being intrinsically much brighter than the sun), Michell’s value was the first realistic estimate of the distance to any star.
Michell was apparently a man of wide interests, including music. Tradition has it that William Herschel was a frequent guest at Thornhill during his years as a young musician in Yorkshire,and he is even said to have received his introduction to mirror grinding from Michell. There is, however, no real evidence that Herschel turned to astronomical observation before his move to Bath some years later; and the story of his apprenticeship with Michell may, therefore, be apocryphal.
BIBLIOGRAPHY
I. Original Works. Michell’s papers appeared mainly in the Philosophical Transactions of the Royal Society and include “Conjectures Concerning the Cause and Observations Upon the Phenomena of Earthquakes,” 51 , pt. 2 (1760),566–634, also published separately (London, 1760); “Observations on the Same Comet [January 1760],” ibid., 466–467; “A Recommendation of Hadley’s Quadrant for Surveying,” ibid., 55 (1765), 70–78, also published separately (London, 1765); “Proposal of a Method for Measuring Degrees of Longitude Upon Parallels of the Equator,” 56 (1766), 119–125, also published separately (London, 1767); “An Inquiry Into the Probable Parallax and Magnitude of the Fixed Stars From the Quantity of Light Which They Afford Us,” ibid., 57 (1767), 234–264, also published separately (London, 1768); and “On the Means of Discovering the Distance, Magnitude, etc. of the Fixed Stars,” ibid., 74 (1784),35–57.
Michell was also author of A Treatise of Artificial Magnets(Cambridge, 1750; 2nd ed., 1751), translated into French as Traité sur les aimans artificiels (Paris, 1752); and De arte medendi apud priscos musices (London, 1766; 1783).
II. Secondary Literature. See Archibald Geikie, Memoir of John Michell (Cambridge, 1918); and Dictionary of National Biography, XIII, 333–334.
ZdenÉk Kopal