The grouping of the stars into constellations is of great antiquity. The exact date of their formation is not exactly known, but an approximate result may be arrived at from the following considerations. On the celestial spheres, or "globes," used by the ancient astronomers, a portion of the southern heavens of a roughly circular form surrounding the South Pole was left blank. This space presumably contained the stars in the southern hemisphere which they could not see from their northern stations. Now, the centre of this circular blank space most probably coincided with the South Pole of the heavens at the time when the constellations were first formed. Owing to the "Precession of the Equinoxes" this centre has now moved away from the South Pole to a considerable distance. It can be easily computed at what period this centre coincided with the South Pole, and calculations show that this was the case about 2700 B.C. The position of this circle also indicates that the constellations were formed at a place between 36° and 40° north latitude, and therefore probably somewhere in Asia Minor north of Mesopotamia. Again, the most ancient observations refer to Taurus as the equinoxial constellation. Virgil says-- "Candidus auratis aperit cum cornibus annum Taurus."[382] This would indicate a date about 3000 B.C. There is no tradition, however, that the constellation Gemini was ever _seen_ to occupy this position, so that 3000 B.C. seems to be the earliest date admissible.[383] Prof. Sayce thinks that the "signs of the Zodiac" had their origin in the plains of Mesopotamia in the twentieth or twenty-third century B.C., and Brown gives the probable date as 2084 B.C.[384] According to Seneca, the study of astronomy among the Greeks dates back to about 1400 B.C.; and the ancient constellations were already classical in the time of Eudoxus in the fourth century B.C. Eudoxus (408-355 B.C.) observed the positions of forty-seven stars visible in Greece, thus forming the most ancient star catalogue which has been preserved. He was a son of Eschinus, and a pupil of Archytas and probably Plato. The work of Eudoxus was put into verse by the poet Aratus (third century B.C.). This poem describes all the old constellations now known, except Libra, the Balance, which was at that time included in the Claws of the Scorpion. About B.C. 50, the Romans changed the Claws, or Chelæ, into Libra. Curious to say, Aratus states that the constellation Lyra contained no bright star![385] Whereas its principal star, Vega, is now one of the brightest stars in the heavens! With reference to the origin of the constellations, Aratus says-- "Some men of yore A nomenclature thought of and devised And forms sufficient found." This shows that even in the time of Aratus the constellations were of great antiquity. Brown says--