Called by Thomson, in his Winter, “the centaur archer.” Anciently, the winter solstice occurred on the entry of the sun into Capricorn; but the stars, having advanced a whole sign to the east, the winter solstice now falls at the sun's entrance into Sagittarius (the centaur archer), so that the poet is strictly right, though we vulgarly retain the ancient classical manner of speaking. Capricornus is the tenth, or, strictly speaking, the eleventh sign of the zodiac. (Dec. 21-Jan. 20.) According to classic mythology, Capricorn was Pan, who, from fear of the great Typhon, changed himself into a goat, and was made by Jupiter one of the signs of the zodiac.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894