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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," 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 a 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 o”.92. In 1835 Struve assigned a doubtful parallax of o”.261 to Vega (a Lyrae). 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 o”.3483; and this was confirmed by Peters, who found the value o”.349. Later determinations for a, Centauri, by Gill,” make its parallax o”.75. This is the nearest known fixed star; and its light takes 44 years to reach us. The light-year 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.” The proper motions and parallaxes combined tell us the velocity of the motion of these stars across the line of sight: a Centauri 14.4 miles a second = 4.2 astronomical units a year; 6.1 Cygni 37.9 miles a second = 1 1.2 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, a. Orionis, a Cygni, 3 Centauri, and r Cassiopeia. Oudemans has published a list of parallaxes observed. Proper Motion. – In 1718 Halley* detected * One of the most valuable contributions to our 1:nowledge of stellar parallaxes is the result of Gill's work (Cape Results, vol. iii., part ii., 1900). * Taking the velocity of light at 186,000 miles a second, and the earth's mean distance at 93,000,ooo miles, I light-year = 5,865,696,ooo, ooo miles or 63,072 astronomical units; I astronomical unit a year = 2.94 miles a second; and the earth's orbital velocity = 18.5 miles a second.
* R. S. Phil. Trans., 181 o and 1817–24.
the proper motions of Arcturus and Sirius. In 1738 J. Cassini' 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 Io". 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. When Huggins first applied the Döppler principle to measure velocities in the line of sight,” the faintness of star spectra diminished the accuracy; but Vögel, in 1888, overcame this to a great extent by long exposures of photographic plates. It has often been noticed that stars which seem to belong to a group of nearly uniform magnitude have the same proper motion. The spectroscope has shown that these have also often the same velocity in the line of sight. Thus in the Great Bear, 3, r, 6, e, o, all agree as to angular proper motion. 6 was too faint for
* Mem. Acad. des Sciences, 1738, p. 337.
a spectroscopic measurement, but all the others have been shown to be approaching us at a rate of twelve to twenty miles a second. The same has been proved for proper motion, and line of sight motion, in the case of Pleiades and other groups. Maskelyne measured many proper motions of stars, from which W. Herschel" came to the conclusion that these apparent motions are for the most part due to a motion of the solar system in space towards a point in the constellation Hercules, R.A. 257°; N. Decl. 25°. This grand discovery has been amply confirmed, and, though opinions differ as to the exact direction, it happens that the point first indicated by Herschel, from totally insufficient data, agrees well with modern estimates. Comparing the proper motions and parallaxes to get the actual velocity of each star relative to our system, C. L. Struve found the probable velocity of the solar system in space to be fifteen miles a second, or five astronomical units a year. The work of Herschel in this matter has been checked by comparing spectroscopic velocities in the line of sight which, so far as the sun's motion is concerned, would give a maximum rate of approach for stars near Hercules, a maximum rate of recession for stars in the opposite part of the heavens, and no effect
* R. S. Phil. Trans., 1783.
for stars half-way between. In this way the spectroscope has confirmed generally Herschel's view of the direction, and makes the velocity eleven miles a second, or nearly four astronomical units a year. The average proper motion of the first magnitude star has been found to be o”.25 annually, and of a sixth magnitude star o”.o.4. But that all bright stars are nearer than all small stars, or that they show greater proper motion for that reason, is found to be far from the truth. Many statistical studies have been made in this connection, and interesting results may be expected from this treatment in the hands of Kapteyn of Groningen, and others." On analysis of the directions of proper motions of stars in all parts of the heavens, Kapteyn has shown “that these indicate, besides the solar motion towards Hercules, two general drifts of stars in nearly opposite directions, which can be detected in any part of the heavens. This result has been confirmed from independent data by Eddington (R.A.S., M.N.) and Dyson (R.S.E. Proc.). Photography promises to assist in the measurement of parallax and proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly * See Kapteyn's address to the Royal Institution, 1908. Also Gill's presidential address to the British