Dividing the Day. Unlike our modern practice of using watches and clocks to keep track of time, the Romans did not have devices that could accurately divide an entire day (our twenty-four-hour period) into smaller parts. Instead, they observed the position of the sun during the day, and with the use of a sundial the Romans could divide the daytime into twelve equal portions called horae (hours). Since the hours of the day during winter are shorter than the hours of the day during summer, the length of a Roman hour fluctuated accordingly. The three principal moments in any day were sunrise, noon, and sunset, and all further subdivision took place between these points in time. The hours of the night were also divided into twelve equal parts.

No Roman "Week." The Roman concept of a "week" was not based...