Let us here take advantage of a parenthesis to explain a few astronomical technicalities, necessary for the due comprehension of our subject.
It is in the plane, or _oblique circle_,--ὁ κῦκλος λοξός, as Ptolemæus called it,--that eclipses occur, owing to the relative positions of the sun, earth, and moon; and it is for this reason modern astronomers have denominated it the _Ecliptic_. The Ecliptic is the Equator of the _oblique sphere_ (σφαῖρα ἐγκεκλιμμένη), properly so called, as the Equator is that of the sphere of the world, or the _right sphere_ (σφαῖρα ὀρθή). The circles parallel to the Ecliptic, which continue to diminish in diameter up to the poles of the oblique sphere, bear the name of _parallels of latitude_; and we give that of _meridians of longitude_ or _oblique ascensions_ (ἀναφοραὶ λοξαί) to the great circles which cut the first rectangularly as they all pass through the axis and the poles of the Ecliptic. The same division by circles cutting each other rectangularly has been made on the right sphere, or sphere of the world. Only, _there_ the latitudes are named _declinations_, and the longitudes _right ascensions_. The general diurnal movement is a movement in right ascension; it is measured upon the Equator. The individual annual movement of the sun is a movement in longitude; it is measured upon the Ecliptic.
The zone, or belt, which the sun seems to trace in its annual march, from the limit of its southern excursion (_the winter solstice_) to the limit of its boreal excursion (_the summer solstice_), and in returning from that limit to the other, after having twice passed through the equinoctial line (or Equator),--this zone is marked on the firmament by a belt of constellations known as the _Zodiac_.
These constellations are named, according to the figurative grouping of the stars (on which we have commented in Book I.),--the Ram, the Bull, the Twins, the Crab, the Lion, the Virgin, the Balance, the Scorpion, the Archer, the Cow, the Water-bearer, the Fishes. There are twelve, three for each season. The constellations represented by these figures, so singularly chosen, spread over the whole celestial vault,--that is, over an extent of 360°.
The heavens, like earth, have their annals: everything changes there as in the human world. In the age of Hipparchus,--or some two thousand years ago,--the sun entered, at the spring equinox, into the zodiacal sign of Aries; in the summer solstice, it entered into that of Cancer; at the autumnal equinox, into Libra; and at the winter solstice, into Capricorn. These signs then corresponded exactly to the constellations which they represent.