Andrew Cooper
Nov 19, 2022    Waikoloa, HI (map)
28cm f/10 SCT, NexStar 11" GyPSy @ 127x
Seeing: 6 Transparency: 7 Moon: 0%

61 Cyg is a lovely wide binary, two rich orange stars separated by about 30", the brighter 61Cyg A is seen to the northwest while 61Cyg B lies to the southeast and about half a magnitude fainter in a rich galactic starfield

Captain William Henry Smyth
Sep 9, 1839    No. 6 The Crescent, Bedford, England (map)
150mm f/17.6 refractor by Tully 1827

A binary star which, from its extraordinary motions, was placed on the Greenwich List, being then the smallest body so honoured. A 5½ and B 6, both yellow, but the small one is of the deepest tint. This most interesting object is H IV 18, and it is formed by Piazzi's Nos. 475 and 476, Hora XX. It is situated on the inner tip of the Swan's right wing, 7°½ south-by-east of Deneb, and nearly on the eastern parallel of Wega.

This star must be regarded as one of the nearest to us, from the great rapidity of its proper motions; and it affords a positive instance of a double star which, besides the individuals revolving round each other, or about their common centre of gravity, has a progressive uniform motion towards some determinate region. This path is relatively spiral, but still so vast as to appear rectilinear; but too little is yet known of its amount and direction to refer it to definite laws. The values, however, have been pretty exactly ascertained:

                  611 Cygni.              612 Cygni.
    Piazzi . . RA +5".38  Dec. +3".30  RA +5".30  Dec. +3".00
    Baily  . .    +5".18       +3".24     +5".28       +3".03
    Argelander    +5".11       +3".23     +5".19       +3".02
    Taylor . .    +5".54       +2".93     +5".52       +3".12
    Myself   .    +5".10       +3".31     +5".21       +3".12
   [Gaia DR2      +4".164174   +3".249991 +4".105786   +3".155759]

and while the journey through interminable space is incessantly performed at this most prodigious rate, the revolving action around each other, at a mean annual rate of +0°.73, had been thus traced previous to my operations, admitting deductions from the meridional observations of Bradley, Christian Mayer, and Piazzi:

    Bradley . . . . .  Pos. 35° 24' Dist. 19".63 Ep. 1753.80
    Mayer   . . . . .       50° 58'       15".24     1778
    Herschel I. . . .       53° 32'       16".08     1780.72
    Piazzi. . . . . .       69° 18'       18".20     1800
    Bessel. . . . . .       79° 07'       16".74     1812.30
    Struve  . . . . .       83° 02'       15".20     1819.90
    Herschel II. and South  84° 41'       15".43     1822.90
    Dawes   . . . . .       90° 20'       15".70     1830.66
   [WDS   . . . . . .      153°           31".80     2019   ]

These data afford much room for reflection. The anomalies are much greater than two stars with 16" between them ought to exhibit, but still there is some allowance to be made in a branch of astronomy which must, even yet, be considered as new. The angles and distances deduced from RAs and Decs, are as accordant as can reasonably be expected under so critical an ordeal; and the infirmity which affects the wire-micrometer, from the inflection of light interfering in the precise contact between the thread and the luminous body, may be admitted in palliation. There is strong presumption that all this will be better managed in future: meantime the average annual motion from the results before us, in the direction sp nf or direct, may be thus roundly tabulated:

    Mayer            +0°.65 1778    | Dawes  +0°.72 1830.66
    Herschel I.      +0°.83 1780.72 | Myself +0°.98 1832.65
    Piazzi . .       +0°.83 1800    |  ———   +0°.43 1834.76
    Bessel . .       +0°.80 1812.30 |  ———   +0°.48 1835.59
    Struve . .       +0°.51 1819.90 |  ———   +0°.73 1837.65
    H. II. and South +0°.57 1822.90 |  ———   +0°.59 1839.69

We here gain a general mean of 60°.9 in eighty-six years, or 0°.71 per annum, whence a revolution of about five centuries may be concluded upon: and while one star is thus going round the other, the pair is journeying through the vastness of space with incomprehensible velocity. Such motions afford a wondrous proof of illimitable power, whether we consider its exertion in the original production of these motions, or in controlling them. So far the vast mechanism of Infinite Nature is before us: but human reason is utterly incapable of speculation upon the end and aim of the UNFATHOMABLE WILL. Vexed at the presumption of ignorance, Milton said:

HE his fabric of the heavens
Hath left to their disputes, perhaps to move
His laughter at their quaint opinions wide.

Piazzi was the first who detected the extraordinary movements of 61 Cygni, as shown at p. 10, 1. vi. of the book Del Reale Osservatorio di Palermo, published in 1806. Yet in 1812, the Moniteur Universel, No. 189, attributes the discovery to Bessel, who indeed had sent an interesting memoir to Baron de Zach's Monatliche Correspondenz. In his Catalogue of 1814 Piazzi makes a reclamation, and Bessel handsomely acknowledged the priority of the Italian astronomer.

Such was the note of preparation which drew attention to the object, and led to that splendid result, SIDEREAL PARALLAX. That two stars should be moving in an apparently rectilinear path at so large a rate, while other stars in the neighbourhood did not appear to be affected with any proper motion, was an incident of promise. By the delicate observations of MM. Arago and Mathieu, in 1812, it appeared that the distance of 61 Cygni was not less than 412,000 times the diameter of the earth's orbit; now, under an annual proper motion of 5", it was hence deduced that the two stars were whirled along at the astounding rate of sixty thousand times faster than Mercury, which is the swiftest moving body in the planetary system.

This, however, was based on the supposition that the observations were perfectly correct; but the lapse of six months, and the change of season, might easily lead to some slight change in the telescope, and the important quantity of a single second would be but the breadth of a spider's thread on the graduated circle of any instrument. Though the argument was insecure, it was a welcome approximation: and thus it remained until Bessel placed a foundation for more precise and definite conceptions of the vast distances of stellar bodies. The means by which he made the signal discovery of parallax, cannot be better given than in his own words, from a letter which he addressed to Sir John Herschel, under date of the 23rd of October, 1838:

I selected among the small stars which surround 61 Cygni, two between the 9th and 10th magnitudes; of which one (a) is nearly perpendicular to the line of direction of the double star; the other (b) nearly in this direction. I have measured with the heliometer the distances of these stars from the point which bisects the distance between the two stars of 61 Cygni; as I considered this kind of observation the most correct that could be obtained, I have commonly repeated the observation sixteen times every night.

The result of his assiduous measurement is, that in summer 61 Cygni was further from a by 0".620 than in winter, and further from b by 0".437. These numbers I derive from a mean of the observations in May, June, and July; and those of November, December, and January. The difference in the amount yielded by the two stars, is accounted for by a being almost at half the distance of b, and in a better angle of position: thus

a Pos. 201° 29' 24" Dist. 461".617
b 109° 22' 10" 706".279

Yet it is only an apparent disproportion, for, after due reduction, the probable value is brought down to 0".3136, according to the following explanation given by the illustrious discoverer :

As the mean error of the annual parallax of 61 Cygni is only +0".0202, and consequently not 1/13 of its value computed; and as these comparisons show that the progress of the influence of the parallax, which the observations indicate, follows the theory as nearly as can be expected considering its smallness, we can no longer doubt that this parallax is sensible.

Assuming it 0".3136, we find the distance of the star 61 Cygni from the Sun 657,700 mean distances of the Earth from the Sun: light employs 10.3 years to traverse this distance. As the annual proper notion of (61) Cygni amounts to 5".123 of a great circle, the relative motion of this star and the Sun must be considerably more than sixteen semi-diameters of the Earth's orbit, and the star must have a constant of aberration of more than 52".

When we shall have succeeded in determining the elements of the motion of both the stars forming the double star, round their common centre of gravity, we shall be able also to determine the sum of their masses. I have attentively considered the preceding observations of the relative positions; but I consider them as yet very inadequate to afford the elements of the orbit. I consider them sufficient only to show that the annual angular motion is somewhere about 2/3 of a degree; and that the distance at the beginning of this century had a minimum of about 15". We are enabled hence to conclude that the time of a revolution is more than 540 years, and that the semi- major axis of the orbit is seen under an angle of more than 15". If, however, we proceed from these numbers, which are merely limits, we find the sum of the masses of both stars less than half the Sun's mass.

For these very delicate researches, the gold medal of the Royal Astronomical Society of London was awarded to M. Bessel: "researches," said Sir John Herschel, "which have gone far to establish the existence and to measure the quantity of a periodical fluctuation, annual in its period, and identical in its law with parallax." When Pliny pronounced the search after solar distance "pene dementis otii est," his mind could not have grasped the conception of stellar parallax.

― A Cycle of Celestial Objects Vol II, The Bedford Catalogue, William Henry Smyth, 1844

Steve Coe

Dbl* 5.2/6m 31 sec PA 150; two red dwarf stars; unique

― SAC Red Star Database

Drawings, descriptions, and CCD photos are copyright Andrew Cooper unless otherwise noted, no usage without permission.

A complete list of credits and sources can be found on the about page