SIR WILLIAM HERSCHEL, F.R.S.—1738-1822.
Painted by Lemuel F. Abbott; National Portrait Gallery, Room XX.
In the field of telescopic discovery beyond our solar system there is no one who has enlarged our knowledge so much as Sir William Herschel, to whom we owe the greatest discovery in dynamical astronomy among the stars—viz., that the law of gravitation extends to the most distant stars, and that many of them describe elliptic orbits about each other. W. Herschel was born at Hanover in 1738, came to England in 1758 as a trained musician, and died in 1822. He studied science when he could, and hired a telescope, until he learnt to make his own specula and telescopes. He made 430 parabolic specula in twenty-one years. He discovered 2,500 nebulæ and 806 double stars, counted the stars in 3,400 guage-fields, and compared the principal stars photometrically.
Some of the things for which he is best known were results of those accidents that happen only to the indefatigable enthusiast. Such was the discovery of Uranus, which led to funds being provided for constructing his 40-feet telescope, after which, in 1786, he settled at Slough. In the same way, while trying to detect the annual parallax of the stars, he failed in that quest, but discovered binary systems of stars revolving in ellipses round each other; just as Bradley’s attack on stellar parallax failed, but led to the discovery of aberration, nutation, and the true velocity of light.
Parallax.—The absence of stellar parallax was the great objection to any theory of the earth’s motion prior to Kepler’s time. It is true that Kepler’s theory itself could have been geometrically expressed equally well with the earth or any other point fixed. But in Kepler’s case the obviously implied physical theory of the planetary motions, even before Newton explained the simplicity of conception involved, made astronomers quite ready to waive the claim for a rigid proof of the earth’s motion by measurement of an annual parallax of stars, which they had insisted on in respect of Copernicus’s revival of the idea of the earth’s orbital motion.
Still, the desire to measure this parallax was only intensified by the practical certainty of its existence, and by repeated failures. The attempts of Bradley failed. The attempts of Piazzi and Brinkley,[[1]] early in the nineteenth century, also failed. The first successes, afterwards confirmed, were by Bessel and Henderson. Both used stars whose proper motion had been found to be large, as this argued proximity. Henderson, at the Cape of Good Hope, observed α Centauri, whose annual proper motion he found to amount to 3".6, in 1832-3; and a few years later deduced its parallax 1".16. His successor at the Cape, Maclear, reduced this to 0".92.
In 1835 Struve assigned a doubtful parallax of 0".261 to Vega (α Lyræ). But Bessel’s observations, between 1837 and 1840, of 61 Cygni, a star with the large proper motion of over 5”, established its annual parallax to be 0".3483; and this was confirmed by Peters, who found the value 0".349.
Later determinations for α2 Centauri, by Gill,[[2]] make its parallax 0".75—This is the nearest known fixed star; and its light takes 4 1/3 years to reach us. The lightyear is taken as the unit of measurement in the starry heavens, as the earth’s mean distance is “the astronomical unit” for the solar system.[[3]] The proper motions and parallaxes combined tell us the velocity of the motion of these stars across the line of sight: α Centauri 14.4 miles a second=4.2 astronomical units a year; 61 Cygni 37.9 miles a second=11.2 astronomical units a year. These successes led to renewed zeal, and now the distances of many stars are known more or less accurately.
Several of the brightest stars, which might be expected to be the nearest, have not shown a parallax amounting to a twentieth of a second of arc. Among these are Canopus, α Orionis, α Cygni, β Centauri, and γ Cassiopeia. Oudemans has published a list of parallaxes observed.[[4]]
Proper Motion.—In 1718 Halley[[5]] detected the proper motions of Arcturus and Sirius. In 1738 J. Cassinis[[6]] showed that the former had moved five minutes of arc since Tycho Brahe fixed its position. In 1792 Piazzi noted the motion of 61 Cygni as given above. For a long time the greatest observed proper motion was that of a small star 1830 Groombridge, nearly 7” a year; but others have since been found reaching as much as 10”.
Now the spectroscope enables the motion of stars to be detected at a single observation, but only that part of the motion that is in the line of sight. For a complete knowledge of a star’s motion the proper motion and parallax must also be known.