Time, Measurements of

Origin of the Day

The length of day was assigned the closest number of periods that fit between solar meridian transits. Mathematicians and archaeologists credit the origins of the time divisions back to the Sumerians, who employed a base-12 counting system. Various theories state that, excluding thumbs, there are 24 segments on the remaining eight human fingers, which Sumerians are believed to have used as a basis for counting. At night, time was measured by the appearance of given stars crossing a certain sky point to mark the time passage. For tracking time during the day, the sundial was invented.

The Babylonians improved this division by segmenting the day into 12 hours (each twice the modern length). Furthermore, the hour was subdivided into 60 units (another base-12 factor), and then divided again by 60 to define the smallest unit. Using factors of 12 kept things simple, and archaeologists know that Babylonians utilized a base-60 (sexagesimal) counting system. The sexagesimal concept has been retained for many centuries because it is simple and reliable. It is no surprise, then, that measurements of geometrical shapes are also in a sexagesimal (extended base-12) system of 360°.

Origins of the Hour, Minute, and Second

When the ancient Greeks required a standard for measuring time, they asked the philosopher Hipparchus to devise a system for planning events. As a result, Hipparchus applied the concept of divisions based on an equinoctial system (equal day and night). This occurred at the equinoxes, and was called Hora, the Greek word from which we derive “hour.” Hipparchus, however, considered only the daylight hours when making these divisions. Ptolemy later defined geometrical divisions of the circle into 60 (minuta) and in turn divided minuta again by 60 (secunda minuta). To this day, clocks are still circular with 60 divisions. Eventually, the geometric system was copied to the measuring of time, and the minute and the second were applied.

The Calendar

Calendars were developed to track the passage of days. The first calendars date back to the time of Mesopotamia, but the Greeks are credited with being the first to attempt to use a standard form of measurement based on the motion of the moon around the earth. The Romans created a separate solar calendar, which became the basis for our modern calendar. Several revisions were made, including adding months to synchronize the lunar and solar cycles, rearranging month lengths, and rearranging monthly delineation. Over time, political issues throughout the empire resulted in an unreliable calendar, and Julius Caesar reorganized the calendar into a standard measurement. The Julian calendar was adopted circa 46 BCE. The lengths of the months in the 12-month Julian calendar remained centered on the lunar cycle, and alternated 30- and 31-day lengths. The only exception was February, which contained only 29 days in order to maintain calendar reliability. Originally, January and February occurred at the end of the calendar year, but eventually the Julian adaptation moved them to the start of the year instead. The result was a misalignment of the names of months with their position in the year: For example, the 7th month, prefixed septem, Latin for “seven,” became the 9th month.

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