Among all peoples and in all ages the most obvious unit for the measurement of time has been the day, and the never-failing reappearance of light after each interval of darkness has been the most constant natural phenomenon with which the mind of man has had to deal. From the earliest times, successive returns of the sun have regulated the whole scheme of human existence. When it was light, the man worked; when it was dark, he rested. Conformity to the operation of this natural law has been practically universal.Indeed, as the primitive man saw nature, a day was the only division of time upon which he could absolutely rely. The waxing and waning of the moon, with its ever-changing shape and occasional obscuration by clouds, as well as its periodic disappearances from the heavens all combined to render that luminary of little account in measuring the passage of time. The round of the seasons was even more unsatisfactory.
