The North Pole and the other, the South Pole, and the meridians are the circles that pass from one pole to the other. They are named thus because if the sun is directly overhead a place through which this line passes at noon, then these circles pass through the center of the Earth. As for the parallels, they are the circles located between the meridians. They are parallel to each other on the Earth's axis, and the greatest of them is the Equator, which is the great circle equidistant from the poles. It divides the sphere into two halves, one the Northern Hemisphere and the other the Southern Hemisphere. Then, the meridians and the parallels, like all circles, are divided into sixty-six degrees, and each degree is divided into sixty minutes, and each minute into sixty seconds, and each second into sixty thirds, and so on. Another new division is that the circle is divided into four quarters, and each quarter is divided into one hundred and six parts, i.e., one hundred and sixty degrees, and each minute is divided into one hundred seconds, and so on. This was established for astronomical and metrical calculations, the former being more famous. These circles determine longitude and latitude. Latitude is the distance of a parallel circle from the great circle, which is the Equator. If taken towards the north, it is northern latitude, ending at ninety degrees. If taken towards the south, it is southern latitude, ending similarly. As for longitude, it is the distance of a meridian from another meridian designated as the prime meridian, which is east and amounts to one hundred and eighty degrees, and west, amounting to the same. Geographers have placed on each parallel circle the degrees of its distance from the Equator, just as they have placed on each meridian the number of degrees of its distance from the prime meridian. This has been drawn as we have previously mentioned.