DSS image of Castor
Overlaid DSS image of Castor, 60' x 60' with north at top and west to the right

Aladin viewer for the region around Castor
Nānāmua, Alpha Geminorum, α Gem, 66 Gem
Σ 1110, BD+32 1581, HD 60178J, WDS J07346+3153, SAO 60198, HIP 36850

Type  Multiple Star
Magnitude  1.58
Right Ascension  7h 34' 35.8"  (2000)
Declination  31° 53' 18" N
Constellation  Gemini
Description  Binary 1.9/3.0/9.2 3"/70"
Classification  A1V+A2Vm
Observing Notes

Andrew Cooper
Feb 19, 2020    Waikoloa, HI (map)
20cm f/6 Newtonian, Cave Astrola @ 61x
Seeing: 7 Transparency: 6 Moon: 0%

Brilliant blue-white pair with about a magnitude difference at 2.0 and 2.9, about 5" separation

Captain William Henry Smyth
Dec 13, 1830    No. 6 The Crescent, Bedford, England (map)
150mm f/17.6 refractor by Tully 1827

A standard Greenwich star, neatly double, in the head of Castor, and about half way between Regulus and Aldebaran. A 3, bright white; B 3½, pale white; C 11, dusky, and there is another very small acolyte at a distance, in the sp quadrant of the field, with which a physical connection has been also suspected. This very interesting object, when classed 1 ♅. II., was thus:
    Pos. 302° 47'  Dist. 5".16  Ep. 1778.27
When H. and S. examined it, the secondary had changed its quadrant from np to sp and stood as follows:
    Pos. 267° 07'  Dist. 5".35  Ep. 1821.21
[WDS 53° 5".40 2020 ]
showing a rapid retrograde motion of 0°.97 per annum; and the more recent observations so fully confirm the binary system, that Sir J. Herschel concludes the small star will pass its periastre at the close of 1855, at a distance of 0".66; which brings its annus magnus to about 250 years. At the moment it was made, this was a bold prediction; and although the state of mutual approach and accelerated angular motion do not circumstantially corroborate it, there is good evidence that the theory is substantially right.

To arrive at his deductions, Sir John Herschel gathered together all the observations he could rely upon, it being a question he was deeply interested in, because, he says, Castor is "the largest and finest of all the double stars in our hemisphere, and that whose unequivocal angular motion first impressed on my father's mind a full conviction of the reality of his long cherished views on the subject of the binary stars." By the alignments of Pound and Bradley, he was able to carry the angle back for upwards of 100 years; and by computations which approximate as near as the present state of the subject will allow, he has deduced the following elements of the elliptic orbit of the secondary round the primary; major semi-axis 8".086; eccentricity 0.7582; position of perihelion 169° 10'; inclination of the real orbit to the apparent orbit on the sphere of the heavens, 70° 03'; mean motion, l°.425; period of revolution 253 years. "This star," he adds, "seems on the point of undergoing, within the ensuing twenty-four years, a remarkable change similar to that of which γ Virginis has already furnished a striking instance during the last century, and passing from a distant double star of the second class to a close one of the first, and ultimately to one of extreme closeness and difficulty, such as only the very finest telescopes, with all the improvements we may expect in them, will be capable of showing otherwise than single." But there are some orbital anomalies still in the way.

Using Herschel's bow, albeit with hardly vigour to bend it, I attempted an orbit of this revolver, notwithstanding I soon found that the values of its annual changes are violently discordant. The projection brought out an ex-centricity of 0.7781, an inclination = 70° 36', and a period of 240 years; the last condition being obtained by H.'s novel and ingenious process of cutting out the graphic orbit from card-board, and weighing both it and its requisite sectors in a balance. These are the previous angles used:
    Bradley and Pound   .  .  .  Pos. 355° 53'  Ep. 1719.84
Bradley and Maskelyne . . 323° 47' 1759.80
Herschel the Elder . . . 293° 03' 1783.64
Herschel the Elder . . . 284° 19' 1800.27
Struve . . . . . . 272° 52' 1813.83
Herschel Junior . . . 270° 00' 1816.97
Herschel Junior and South . 264° 59' 1823.11
Several years previous to the combined operations in the Blackman-street observatory, Sir John Herschel had measured various double stars at Slough, with a 7-foot reflector; and as some of the synoptic results only were printed in the register published by him and Sir James South, in 1824, I requested of him, and obtained, the full details from his journal. With the accustomed diffidence of real merit, Sir John places "no vast confidence" in his observations of 1816, except in the above-cited measurement of Castor, "which," he tells me, "from the circumstances described, must be correct, and is valuable. And I well remember comparing (with my father) this particular result with his former measures, much to his and my own satisfaction, as a verification of its orbital movement." The circumstances alluded to are thus given in the Journal: "Dec. 20, 1816, α Geminorum. Double. Unequal. White—both stars. Position of the small star, exactly preceding—I made both stars run along the wire repeatedly from one side to the other, and both were covered. I then made them run above and below it, and could perceive no deviation from exact parallelism. The evening is perfectly still, clear, and frosty."

Bradley appears to have made his estimations upon the parallelism of the line of direction of the pair, to that joining Castor and Pollux, in 1759, "at all times of the year," evidently intending to notice whether any annual oscillation might be observed. This induced Roger Long, Lowndes's Professor of Astronomy at Cambridge, to attack Castor with telescopes of fourteen and seventeen feet in length, with a view to Galileo's suggestion on parallax; but the prospect of success soon became so hopeless, that he was "persuaded the stars would always be found to appear the same." This gentleman was more happy in the construction of an enormous astronomical machine—the very A1 of orreries—at Pembroke College. It is a hollow sphere, about eighteen feet in diameter, with its polar axis parallel to the mundane axis, upon which it is readily turned by a winch and rack-work; thus it can be made to revolve, while about thirty persons conveniently attend a scientific lecture in the interior, and contemplate the orderly march of the constellations painted on the moving concavity above them, the stars being pierced through the metal according to the several magnitudes, so that the light penetrates and each assumes a curious radiated, or rather stellated form. This sphere was completed, with considerable expense as well as ingenuity, in 1758; but although six pounds per annum is allowed to a keeper, who is generally an under-graduate, it was suffered to fall so much out of order as to mar the projector's intention of popularizing astronomy; and many a good man and true has lived and learned in Cambridge, without even being aware of its existence. Of this I could tell a story or two, but shall only add, that it was lately brushed up a bit; and I had the satisfaction of being on its floor with a party of Cambridge savans of the first magnitude, in whom the shade of Long must have delighted.

Διδνμοι, Gemini, Tindaridaæ, or Gemelli, is the third constellation of the zodiac, and one of the ancient 48; lying nearly mid- way between Orion and the Great Bear, in a region long viewed as the centre of the heavens. Among the Orientals it was represented as a pair of kids, denoting that part of spring when these animals appear; but the Greeks changed them to two children with their feet on the Galaxy; and the Arabians, whose tenets prohibited the human form in delineations, afterwards altered them to a couple of peacocks. Paulus Yenetus, and the early Venetian illustrators of Hyginus, represent them as two winged angels. Among the ancients every sign had its tutelary deity, and Phœbus had charge of Gemini, which gave rise to the astrological jargon about the connexion between the sun and this asterism; to the disparagement of the latter, for many inuendos are on record, and we are told, in the manuscript Almanac of 1386, that whoever happens to be born under the aërial triplicity of the Twins, shall be "ryght pore and wayke, and lyf in mykul tribulacion."

Astronomers, however, view it in a different light; for though it is not splendidly conspicuous nor thickly studded, it is fine, and contains bright individuals, which, with its numerous double stars, clusters, and nebulæ, render it interesting and important; and, from its being the sign of St. Paul's ship, we see that it was esteemed propitious by ancient mariners. It has been thus tabulated:
    Ptolemy   . . . 25 stars  Bayer    . . .  33 stars
Copernicus . . 25 Hevelius . . . 38
Tycho Brahé . . 29 Flamsteed . . 85
Kepler . . . 30 Bode . . . . 190
Castor was called by the later Arabians, Rás-al-tawum al-mokaddem, the head of the foremost twin; and with Pollux it constitutes al-dhirá' al-mebsútah, the outstretched arm, forming the VIIth Lunar Mansion. This dhira' is intended to mean the drawn-in paw of the large lion alluded to by Kazwíní; an allusion which Ideler ascribes rather to the disciples of confusion and ignorance, than to astrognosts. This huge monstrosity may be thus figured. The two stars in the heads of the Twins, and in the Lesser Dog, form its paws, the Præsepe, its nose— ζ, γ, η, and α in the Greek Lion, its forehead—Arcturus and Spica, its shin-bones β, η, γ, δ, and ε Virginis, the hips—and the stars in Corvus, its hind quarters. No wonder that Ideler indignantly asks, who could have made such a mistake as placing the nose on the forehead, the legs on the hip? "Welches Missverhältniss! Der Nase zur Stirn, der Schienbeine zur Hüfte!"

To know this star by alignment is easy, as a ray from Rigel, led through ε, the middle star of Orion's belt, and under Betelgeuze, will, at about twice that distance further on, rest upon Castor: or, if taking the poetaster's advice:
    From gamma on the Great Bear's flank    let a long ray be cast,
Conduct it under Merak's blaze to south-west regions vast;
Across the Lynx to Gemini this line will thus be led,
And carried further on will reach bright Betelgeuze the red.
A proper motion in space has been assigned to the principal star, to the following amount:
    P....  RA -0".16  Dec. -0".10
B.... -0".12 -0".07
A.... -0".21 -0".08
[Hipparcos -0".19145 -0".14519]
― A Cycle of Celestial Objects Vol II, The Bedford Catalogue, William Henry Smyth, 1844
Other Data Sources for Castor
Associated objects for Castor
Nearby objects for Castor

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

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