Thus we know that God created this world in six days. For that reason the tongues of those who are unfamiliar with the prophets’ laws—such as the pagan Turks and others—contain no names for the days of the week; they simply do not know it. Custom follows perception: whoever has no conception of a thing cannot name it.

A day is marked by the rising of dawn, the light that appears from the eastern horizon. It is the first continuous glow of the sun, uninterrupted—unlike the false dawn, after which darkness returns. In the Sacred Law the time for prayer, fasting, and other acts is tied to this second light. It is recognized by direct sensory observation just as the crescent moon is, and it may also be estimated: once the positions of the stars at dawn are noted, they can be used on the following day to infer the moment of its rise.

Assigning the span of dawn a fixed portion of the celestial motion equal to that of the evening twilight, as some time-keepers have done, is an error—no different from those who posit a universal measure for the arc of lunar visibility. Dawn is the sun’s light, that is, its rays reflected by the air and the earth, and this necessarily varies with the surface on which the rays fall. When the sky is clear of clouds, the light does not show as it does when vapor is present; owing to its thickness and density, vapor reflects the rays in a way that the thin air cannot. Observe that when the sun rises its beams first become visible on dense bodies—earth, mountains, and the like—and that if those bodies are polished, like mirrors or water, the reflection is even brighter. As for the air, although it is illumined, the ray does not stay within it; it passes through until it meets a dense body and is reflected.

In winter, nighttime vapors are abundant because the earth’s moisture rises and the cold prevents the vapor from dispersing; the rays therefore rebound from it, and dawn appears earlier than it would if no vapor were present. In summer, by contrast, the daytime sun disperses the vapor, so when it sets there is nothing to reflect its trailing rays, and the interval of the ʿIshāʾ twilight in summer becomes correspondingly longer.