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The three most familiar divisions of time we make today continue to be based on the motions of heavenly bodies, though now we call this astrophysics. The length of a year is determined by the time it takes the earth to circle the sun; the length of a month is (more or less) the time it takes the moon to circle the earth; the length of a day is the time it takes the earth to rotate on its axis (and observed by us as the span between two successive sunrises or sunsets). But further divisions are not based on any physical laws and tend to be based on historical factors that are largely arbitrary. There is nothing inherent in any biological or astrophysical cycle that would lead to the division of a day into twenty-four equal segments.