Castor, Ovid's Eques, the Horseman of the Twins, and the mortal one as being the son of Tyndarus, is the well-known name for this star, current for centuries; but in later Greek days it was Ἀπόλλων, and Apollo with the astronomers even through Flamsteed's time.
It will be remembered that till toward the Christian era this name for the god of day was the title of the planet Mercury when morning star, its rapid orbital movement and nearness to the sun preventing its earlier identification with the evening star, which was designated, as now, after the gods of thieves and darkness. In Percy's Reliques Mercury is described as "the nimble post of heaven"; Goad, in 1686, called it
a squirting lacquey of the sun, who seldom shows his face in these parts, as if he were in debt;
while this same quick motion induced the alternative word of the chemists for quicksilver, as well as for the very uncomfortable human temperament that Byron described:
a mercurial man
Who fluttered over all things like a fan.
Notwithstanding, however, the supposed difficulty of seeing Mercury, — Copernicus died regretting that he had never observed it, although this was doubtless partly due to his high latitude and the mists arising from the Vistula at Thorn, — the canon Gallet, whom La Lande styled Hermophile, saw it 100 times, and Baily said that Hevelius observed it 1100 times! Indeed, it is easily visible in the latitude of New York City for several days, at its elongation, if one knows where to look for it.
But to return to our star Castor.
It was Ἀπέλλων in the Doric dialect, which degenerated into Afelar, Aphellon, Aphellan, Apullum, Aphellar, and Avellar; the Avelar of Apian2 of the 16th century subsequently appearing as Anelar, the Alfonsine Anhelar.
Caesius had the synonymous Phoebus, and also cited Theseus, but this should rather be applied to β as another title of the original Hercules. Bayer gave Rasalgeuze; and Riccioli, Algueze vel potius Elgiautzi, but these also better belong to β.
The Babylonians used Castor to mark their 11th ecliptic constellation, Mash-mashu-Mahrū, the Western One of the Twins; while with Pollux the two constituted Mas-tab-ga-gal-gal, the Great Twins. In Assyria they were Mas-mas and Tuāmu, the twins, although that country knew other twin stars here as well as elsewhere in the sky. As an object of veneration Castor was Tur-us-mal-maχ, the Son of the Supreme Temple; but in astrology, everywhere, it has been a portent of mischief and violence.
When the Arabians adopted the Greek figures they designated this star as Al Rās al Taum al Muḳaḍḍim, the Head of the Foremost Twin; but, according to Al Tizini, the early and indigenous term was Al Awwal al Dhirāʽ, the First in the Paw or Forearm. Reference was made by this to the supposed figure of the enormous early Lion, the nomads' Asad, the Outstretched Forearm of which α and β marked as Al Dhirāʽ al Mabsuṭāt. This extended still further over Gemini, the other, the Contracted one, Al Maḳbūḍah, running into Canis Minor. The rest of this monstrosity included Cancer, part of our Leo, Boötes, Virgo, and Corvus, as was mentioned by Kazwini, and commented on by Ideler, who sharply criticized mistakes in its construction. Al Bīrūnī also described this ancient figure, especially complaining of the many errors and much confusion in the Arab mind as to the nomenclature of the two stars, although he himself used titles for them generally applied only to Sirius and Procyon. Ideler and Beigel attributed this exaggerated and incongruous formation to blunders of misunderstanding and transcription by early writers and copyists. Indeed, the former asserted that the whole was the creation of grammarians who knew nothing of the heavens, and arbitrarily misrepresented older star-names.
The two bright stars were the 5th manzil, Al Dhirāʽ, and the 5th nakshatra, Punarvarsū, the Two Good Again; Aditi, the sky goddess, mother of the Adityas, being the presiding divinity, and β marking the junction with Pushya, the next nakshatra. They also constituted the 5th sieu, Tsing, a Well, or Pit, anciently Tiam, although this was extended to include ε, d, ζ, λ, ξ, γ, ν, and μ, Biot making the last the determinant star.
α and β also were a distinct Chinese asterism, Ho Choo, and with γ and δ were Pih Ho.
As marking lunar stations, Brown thinks them the Akkadian Supa, Lustrous; the Coptic Pimafi, the Forearm; the Persian Taraha, the Sogdian Ghamb, and the Khorasmian Jiray, — these last three titles signifying the Two Stars. Hyde wrote that the Copts knew it as Πιμάι, or Πιμαίντεκεων, the Forearm of the Nile; κεων being for Gihon, a name for that river.
Castor is 7° north of the ecliptic, but, although literally heading the constellation, is now fainter than its companion, and astronomers generally are agreed that there has been inversion of their brilliancy during the last three centuries. It culminates on the 23d of February.
It is among
those double stars
Whereof the one more bright
Is circled by the other,
viewed by the Self-indulgent Soul of Tennyson's Palace of Art; and Sir John Herschel called it the largest and finest of all the double stars in our hemisphere; while the rapid revolution of its two components first convinced his father of the existence of binary systems. But Bradley had already noticed a change of about 30° in their angle of position between 1718 and 1759, and "was thus within a hair's breadth of the discovery of their physical connection," afterwards predicted, in 1767, by the Reverend John Mitchell, and positively made in 1802 by Sir William Herschel, who coined the word "binary" now applied to this class of stars. Burnham wrote in 1896 that we have only 36 pairs whose orbits can be said to be well determined, and about 230 other pairs probably binary systems; and there are 1501 other pairs, within 2ʺ of space between the components, from which the foregoing number may be increased; as well as other pairs now known only as having a common proper motion. Of course the stars observed till now have been almost entirely in the northern heavens, — within 120° of the pole, — so that these numbers may be largely added to as astronomers turn their attention to the southern skies with this object in view.
The orbit of Castor is such, however, that the observations of even a century do not enable us to calculate its size or period with any certainty; but the period certainly is long, — probably between 250 and 1000 years. The components at present are about 5ʺ.7 apart, equal to the angle subtended by a line an inch long at the distance of half a mile. Their position angle is about 227°.
The spectrum is of the Sirian type, and, according to the Potsdam observers, the star is approaching us at the rate of 18.5 miles a second. In 1895 Belopolsky announced that the larger star, like Spica, is a spectroscopic binary, completing its revolution in less than three days around the centre of gravity between it and an invisible companion, with a velocity of about 15 1/2 miles a second.
Burnham thinks that the 9.5‑magnitude star, 73ʺ distant, forms, with the two larger, one vast physical system.
In 1888 Barnard found five new nebulae within 1° of Castor.
is Pollux, formerly Polluces, the Greek Πολυδευκής; Ovid's Pugil, the Pugilist of the Two Brothers, and the immortal one as being son of Zeus.
As companion of Ἀπόλλων, this was Ἡρακλῆς and Ἡρακλέης, descending to Flamsteed's day as Hercules, and degenerating, in early catalogues, into Abrachaleus, that Caesius derived from the Arabic Ab, Father, and the Greek word; this being contracted by some to Aracaleus, by Grotius to Iracleus, by Hyde to Heraclus, and by Riccioli to Garacles. All these are queer enough, as are some of Castor's titles; but what shall be said of Riccioli's Elhakaac, that he attributes to the Arabs for α and β jointly, and Ketpholtsuman for β alone, and with no clue to their origin!
It was the early Arabs' Al Thānī al Dhirāʽ, the Second in the Forearm; but the later termed it Al Rās al Taum al Muʼaḣḣār, the Head of the Hindmost Twin, and Al Rās al Jauzāʼ, the Head of the Twin, — the Alfonsine Rasalgense and Rasalgeuze, that elsewhere is Rasalgauze. Riccioli cited Elhenaat, but this he also more properly gave to γ.
β was the determinant of the 12th Babylonian ecliptic asterism Mash-mashu-arkū, the Eastern One of the Twins; and individually Mu-sir-kes-da, the Yoke of the Inclosure.
It lies 12° north of the ecliptic, the zodiac's boundary line running between it and Castor; and Burnham has found five faint companions down to 13.5 magnitude.
Elkin gives its parallax as 0ʺ.057; and Scheiner, its spectrum as Solar; its rate of recession from us being about one mile a second.
It is one of the lunar stars made use of in navigation; and, in astrology, differed from its companion in portending eminence and renown.
Ptolemy characterized β as ὑπόκιρρος, a favorite word with him for this star-tint, and generally supposed to signify "yellowish" or "reddish," Bayer correctly following the former in his subflava; but the Alfonsine Tables of 1521 translated it quae trahit ad aerem, et est cerea. Miss Clerke, somewhat strongly, says "fiery red."
The two lucidae probably bore the present title of the constellation long antecedent to the latter's formation; they certainly were the Mas-mas, or twins, of the Assyrians, independent of the rest of the figure.
As a convenient measuring-rod it may be noted that α and β stand 4 1/2° apart; and this recalls an early signification of their manzil title, Al Dhirāʽ, the Arabs' Ell measure of length that the stars were said to indicate. This naturally became the dual Al Dhirāʼān that also was used on the Desert for other similar pairs of stars.