The Sphere of the World with Commentaries

limb makes a semicircle and not a perfect circle; so the colure always appears to us imperfect, since only one of its halves appears, while the other is hidden from us. The colure, therefore, distinguishing the solstices, passes through the poles of the world, through the poles of the zodiac, and through the maximum declinations of the sun; that is, through the first degrees of Cancer and Capricorn. Whence the first point of Cancer, where this colure intersects the zodiac, is called the point of the summer solstice, because when the sun is in it, it is the summer solstice, and the sun cannot approach closer to the zenith of our head. The zenith is, however, the point in the firmament placed directly above our heads. The arc of the colure, indeed, which is intercepted between the point of the summer solstice and the equinoctial, is called the maximum declination of the sun. And it is, according to Ptolemy, 23 degrees and 51 minutes. According to Almeon, however, 23 degrees and 33 minutes. Similarly, the first point of Capricorn, where the same colure intersects the zodiac from the other side, is called the point of the winter solstice; and the arc of the colure intercepted between that point and the equinoctial is called the other maximum declination of the sun, and it is equal to the former. The other colure, indeed, passes through the poles of the world and through the first points of Aries and Libra, where there are two equinoxes; whence it is called the colure distinguishing the equinoxes. These two colures intersect one another upon the poles of the world at right spherical angles. The signs of the solstices and equinoxes are evident from these verses: "These two solstices Cancer and Capricorn make, but Aries and Libra make the nights equal to the days."

CONCERNING THE MERIDIAN AND HORIZON.

There are, again, two other greater circles in the sphere, namely the meridian and the horizon. The meridian is a certain circle passing through the poles of the world and through the zenith of our head. And it is called the meridian because wherever a person may be, and at whatever time of the year, when the sun, by the motion of the firmament, arrives at its meridian, it is noon for that person. For a similar reason it is called the circle of mid-day. And it is to be noted that cities, one of which approaches more to the east than another, have different meridians. The arc of the equinoctial intercepted between two meridians is called the longitude of the cities. If, however, two cities have the same meridian, then they are equally distant from the east and west. The horizon, indeed,