I say: Day and night (i.e. νυχθήμερον) are one revolution of the sun in the rotation of the universe, starting from and returning to a circle, which has been assumed as the beginning of this same Nychthemeron, whichsoever circle it may be, it being determined by general consent. This circle is a "great" circle; for each great circle is dynamically an horizon. By "dynamically" (τῇ δυνάμει), I mean that it (this circle) may be the horizon of any place on the earth. By the "rotation of the universe" I mean the motion of the celestial sphere, and of all that is in it, which we observe going round on its two poles from east to west.
The Setting of the Sun as the beginning of the Day.—Now, the Arabs assumed as the beginning of their Nychthemeron the point where the setting sun intersects the circle of the horizon. Therefore their Nychthemeron extends from the moment when the sun disappears from the horizon till his disappearance on the following day. They were induced to adopt this system by the fact that their months are based upon the course of the moon, derived from her various motions, and that the beginnings of the months were fixed, not by calculation, but by the appearance of the new moons. Now, full moon, the appearance of which is, with them, the beginning of the month, becomes visible towards sunset. Therefore their night preceded their day; and, therefore, it is their custom to let the nights precede the days, when they mention them in connection with the names of the seven days of the week.
Those who herein agree with them plead for this system, saying that darkness in the order (of the creation) precedes light, and that light suddenly came forth when darkness existed already; that, therefore, that which was anterior in existence is the most suitable to be adopted
