Commentary Purg IV 61-66

Virgil's explanation of the position of the sun in the morning sky may be paraphrased as follows: If the sun (the mirror), which moves from one side of the equator to the other, were in the constellation Gemini (Castor and Pollux, the celestial twins) and not Aries (where it is now -- see [Inf I 37-40]), Dante would see the sun's path (the red part of the zodiac) as close to the Bears (Ursa Major and Ursa Minor) and thus as far north as it ever gets (at the summer solstice, 21 June).  It would do this, Virgil adds, in an apparently gratuitous detail (but see the next passage), unless it were to veer from its ordained path (which of course it will not in any normal expectation).

      'Zodiac, a belt of the heavens eighteen degrees in breadth, extending nine degrees on either side of the Ecliptic, within which, according to the Ptolemaic system, the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn perform their annual revolutions.  It is divided into twelve equal parts of thirty degrees, called signs, which are named from the constellations lying within them.'  (See zodiaco.)