Measuring time - Common Units of Time
“To fill the hour, that is happiness; to fill the hour, and leave no crevice for a repentance or an approval.” Ralph Waldo Emerson
As calendars become more organized and clocks more precise, new units of measure are continually being added to the list of ways that we use to describe the passage of time. In the beginning, units were measured strictly by observing the behaviors of solar bodies – the earth, the sun, and the moon – and recording the time associated with their cyclic patterns. Now we can, if we so choose, break away from the limitations imposed on us by our solar system, and record time in ways that we find more appropriate, whether they correlate with the movement of celestial bodies all about us or not.
A Year, in its simplest form, is the amount of time that it takes for the earth to revolve once around the sun. Specifically, a year is three-hundred-sixty-five days, six hours, nine minutes, and 9.54 seconds. Therefore, a year is not exactly three-hundred-sixty-five days nor is it quite three-hundred-sixty-six. As it turns out, there are actually a number of ways to define a year. Although the above definition is the one most commonly used, it is actually the definition of a sidereal year – the time it takes the earth to rotate about the sun, relative to the stars. A Tropical year on the other hand, measures the time between two vernal equinoxes, or the moment when the sun seems to be crossing the equator. Its length is three-hundred-sixty-five days, five hours, forty-eight minutes and forty-six seconds. Other commonly used definitions of a year include: anomalistic year, eclipse year, and solar year.
According to the calendar of the French Revolution, a decade in those days was ten days long. Today, a decade is a ten-year period, a century is one hundred years, and a millennium is one thousand years. There have traditionally been two different viewpoints concerning when a century or millennium ends. The Gregorian calendar does not technically have a year zero, and so counting really begins in the year 1 AD. In that fashion, the third millennium began on January 1st, 2001, because the first millennium would have begun in the year 1 AD and the second would have begun in 1001 AD. Popular opinion, however, states that a new decade, century, or millennium begins when the “zeros” roll over, and therefore 1990 – 1999 is considered a decade, 1900 – 1999 is considered a century, and 1000 – 1999 is considered a millennium, with the next one beginning on January 1st, 2000. Though not as commonly used, a score is a measurement of two decades, or twenty years, and a lustrum is a way to describe half a decade, or five years.
The month is another naturally occurring unit of time. Astronomers began quantifying this span of time while they observed the moon cycle through its phases. One month is the time it takes the moon to circle the earth and pass through all of its phases. Though the concept of a month takes on religious overtones in some eastern religions, in the west it is simply considered to be a convenient way to divide the year and seasons. Its artificial counterpart is the quarter. A quarter, or three months, is primarily used to divide the fiscal year into four manageable parts for the purpose of calculating profits, losses, and taxes.
The week, though certainly common in today’s society, stems from the habits of man. The seven-day week seems to have Jewish origins, the seventh day being significant for its mystical meaning in relation to the Holy Sabbath. Other cultures defined weeks spanning from four days to as long as ten days, whatever was a suitable interval between market times. Somehow, the seven-day week stuck. From there, the fortnight, or measurement for two weeks, (fourteen days) was established.
The day is the smallest unit of time that still derives its real meaning from the earth's rotation. A day, or the amount of time it takes for the earth to rotate once on its axis, has always been the most useful and direct way to monitor the passage of time.
From there, time has been divided into increasingly smaller increments; the day is split into twenty-four hours, the hour into sixty minutes, and the minute into sixty seconds. Work days, lunch hours, and bed times are all regulated by the hour, despite the fact that it is an entirely man made concept and has been interpreted differently over time within different societies. The French divided the day into ten hours rather than 24 in the late seventeen hundreds. Prior to hours, tides were the time indicators of choice. The minute really came into play around the time of the industrial revolution when work shifts and train schedules needed detail. Prior to that, clocks had no minute hand.
The second is the base unit of time in the International System of Units, and the commonly known equivalent of one sixtieth of a minute. From microseconds, or millionths of a second, to nanoseconds, billionths of a second, the universe seems to be getting smaller as we endlessly hone in on the smallest moments of life as time passes by. A moment, on some Arabic calendars, denotes one sixtieth of a second in the same way that a second denotes one sixtieth of a minute in English time. How difficult it must be to capture the essence of a moment.
Perhaps one of the most abstract units of time in general use today is the concept of a generation. It is often though of as being synonymous with era or more specifically, the lifespan of a person and his siblings. The greatest generation, the postwar generation, generation x – all of these refer to people born roughly within thirty years of each other who are thought to share the same values and ideals, and have experienced the same hardships. Although equally applicable to plants and animals, the relationships between generations in humans, and the feeling of coexistence that members of one generation feel with each other as well as with past and future generations, helps to mold the human existence into one of tradition and culture.
- Aveni, Anthony. Empires of Time. NY: Basic Books, 1989.
- Goudsmit, Samuel and Robert Claiborne. Time. NY: Life Science Library, 1980.