The clepsydra, or water clock, was also used to record time during the night, and was perhaps the most accurate timekeeping device of the ancient world. During the New Kingdom (1550 to 1070 B.C.), this measuring system was simplified to use a set of 24 stars, 12 of which marked the passage of the night. The period of total darkness was marked by the remaining 12 stars, again resulting in 12 divisions of night (another nod to the duodecimal system). The passage of night could be marked by the appearance of 18 of these stars, three of which were assigned to each of the two twilight periods when the stars were difficult to view. During the era when sundials were first used, however, Egyptian astronomers also first observed a set of 36 stars that divided the circle of the heavens into equal parts. Without the aid of sundials, dividing the dark interval between sunset and sunrise was more complex than dividing the sunlit period. Without artificial light, humans of this time period regarded sunlit and dark periods as two opposing realms rather than as part of the same day. Although the hours within a given day were approximately equal, their lengths varied during the year, with summer hours being much longer than winter hours. The next-generation sundial likely formed the first representation of what we now call the hour. This division reflected Egypt's use of the duodecimal system-the importance of the number 12 is typically attributed either to the fact that it equals the number of lunar cycles in a year or the number of finger joints on each hand (three in each of the four fingers, excluding the thumb), making it possible to count to 12 with the thumb. A T-shaped bar placed in the ground, this instrument was calibrated to divide the interval between sunrise and sunset into 12 parts. As early as 1500 B.C., the Egyptians had developed a more advanced sundial. The first sundials were simply stakes placed in the ground that indicated time by the length and direction of the resulting shadow. Thanks to documented evidence of the Egyptians' use of sundials, most historians credit them with being the first civilization to divide the day into smaller parts. The civilizations that first divided the day into smaller parts, however, used different numeral systems, specifically duodecimal (base 12) and sexagesimal (base 60). In today's world, the most widely used numeral system is decimal (base 10), a system that probably originated because it made it easy for humans to count using their fingers. Lombardi, a metrologist in the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colo., takes the case. History/origin: The term "minute" is derived from the Latin "pars minuta prima" which means the "first small part." The minute was originally defined as 1/60 of an hour (60 seconds), based on the average period of Earth's rotation relative to the sun, known as a mean solar day.Ĭurrent use: The minute, as a multiple of the second, is used for all manner of measurements of duration, from timing races, measuring cooking or baking times, number of heart beats per minute, to any number of other applications.Michael A. Under Coordinated Universal Time, a minute can have a leap second, making the minute equal to 61 rather than 60 seconds. Minuteĭefinition: A minute (symbol: min) is a unit of time based on the second, the base unit of the International System of Units (SI). Durations starting from 30 minutes (half an hour) are commonly used colloquially. Most forms of employment require a certain number of hours worked per day, with compensation being based on the measured or expected hours worked. It is believed that the custom of dividing the cycle of day and night into 24 periods began with the ancient Egyptians.Ĭurrent use: The hour is used globally as a unit of time. There have been many definitions of the hour throughout history based on culture or region, such as the hour being divided into 1/12 of a period daylight or darkness. History/origin: The term "hour" is derived from the Anglo-Norman "houre," which was in use around the 13 th century. In Coordinated Universal Time, a negative or positive leap second may be incorporated to keep the hour within 0.9 seconds of universal time, making an hour either 3,599 or 3,601 seconds in some cases. Definition: An hour (symbol: h) is a unit of time conventionally defined as 1/24 of a day and 3,600 seconds.
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