The Sphere of the World

...since only one half of it appears: the other, however, is hidden from us.

The colure a great circle on the celestial sphere that passes through both poles distinguishing the solstices thus passes through the poles of the world, through the poles of the zodiac, and through the points of the sun's maximum declination the angular distance of a heavenly body north or south of the celestial equator—that is, through the first degrees of Cancer and Capricorn. Wherefore the first point of Cancer, where this colure intersects the zodiac, is called the point of the summer solstice the point where the sun reaches its highest northern point, creating the longest day; because when the sun is in it, it is the summer solstice, and the sun cannot approach any closer to our zenith. The zenith is a point in the firmament directly placed above our heads. The arc of the colure, however, which is captured between the point of the summer solstice and the equinoctial the celestial equator, is called the maximum declination of the sun. And it is, according to Ptolemy Claudius Ptolemy (c. 100–170 AD), the most influential astronomer of antiquity, 23 degrees and 51 minutes. According to Almeon The Caliph Al-Ma'mun (reigned 813–833 AD), who commissioned new astronomical measurements in Baghdad, however, it is 23 degrees and 33 minutes.

Similarly, the first point of Capricorn, where the same colure intersects the zodiac on the other side, is called the point of the winter solstice the point where the sun is furthest south, creating the shortest day; and the arc of the colure captured between that point and the equinoctial is called the other maximum declination of the sun, and it is equal to the first one.

The other colure indeed passes through the poles of the world and through the first points of Aries and Libra, where the two equinoxes the two times each year when day and night are of equal length occur; wherefore it is called the colure distinguishing the equinoxes. These two colures intersect each other at the poles of the world at right spherical angles. The signs of the solstices and equinoxes are made clear by these verses: These two solstices are made by Cancer and Capricorn; but Aries and Libra make the nights equal to the days. original Latin: "hec duo solsticia faciūt cācer capricor:n⁹ sed noctes equāt aries ⁊ libra diebus."

Concerning the meridian and the horizon.

There are also two other major 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 our zenith. And it is called "meridian" because wherever a person may be, and at whatever time of year, when the sun reaches its meridian by the motion of the firmament, it is midday Latin: meridies for that person. For a similar reason, it is called the "circle of mid-day." It should be noted that cities, where one is further to the east than the other, have different meridians. The arc of the equinoctial captured between two meridians is called the longitude of cities geographic longitude. If, however, two cities have the same meridian, then they are equally distant from the east and the west. The horizon, moreover—