Time is the passage from past to present to future. The present is what people experience as now. The present is the time in which peoples' consciousness exist. The past is what comes before now. The past is what people remember. The future is what comes after now. People can only guess at the events of the future.
Time can not be seen or touched. Time is only marked by changes we note from one time period to the next. For example, the setting and rising of the sun marks the passage of a day. The seasons, caused by the tilt of the earth, mark the passage of a year. Time can be measured by the changes people can observe.
One way time differs from other things that we measure is that time goes in only one direction, from the past towards the future. In space, one can go left or right, forward or backward, or up and down. Time only goes forward. Time is unidirectional.
The International System of Units, a standard defining units of measure, defines time in terms of the frequency of light given off by the cesium atom in a certain state transition. Each cycle of the wave is one event that is observed. A second is defined as the passage of 9,192,631,770 such events. The formal definition of a second is:
The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.Sciences such as physics and chemistry use the second as a base unit of measure. Sciences such as biology, that deal with living organism and their circadian cycles, use the day or the year as a base unit of measure.
|Unit of Measure||Abbreviation||Definition(s)||Use|
|nanosecond||ns||1 ns = 10-9 s|
1 million ns = 1 s
|The nanosecond is often used as a time interval on which atomic events are measured.|
|millisecond||ms||1 ms = 10-3 ds|
1000 ms = 1 s
|The millisecond is used to measure events of very short duration, such as instruction cycles in a computer.|
|second||s||1000 ms = 1 s|
60 s = 1 min
|The second is the standard unit of measure for all measurements.|
|minute||m, min||60 s = 1 min|
60 min = 1 hour
|The minute is often used to measure personal events of short duration, such as "I'll be there in five minutes."|
|hour||h, hr||60 min = 1 hr|
3600 s = 1 hr
24 hr = 1 day
|The hour has two definitions that are not exactly the same. 1 hour equals 3600 seconds and and 24 hours equals one day. However, because of slight variations in the rotation of the earth, the length of one day is not always the same. Which definition of an hour is used depends on whether the second or the day is the base unit of measure.|
|quarter-hour||none||1 quarter-hour = 15 minutes|
4 quarter-hours = 1 hour.
|A quarter-hour is usually used to approximate a time of days, such as "It is quarter to four," meaning the time is fifteen minutes before 4, or 3:45.|
|half-hour||none||1 half-hour = 30 minutes|
2 half-hours = 1 hour.
|A half-hour is usually used to approximate a time of day, such as "It is half past seven," meaning the time is 30 minutes past seven, or 7:30.|
|day||dy||1 day = 24 hrs|
7 days = 1 week
|1 day is the time it takes for the earth to rotate once on its axis. Since the rotation of the earth fluctuates slightly, this is not an exact time in seconds.|
|week||wk||1 week = 7 days|
4.3 weeks ≈ 1 month
52 weeks ≈ 1 year
|A week is seven days. The week originated in Jewish culture, as described in the book of Genesis in the Bible. The week is now universally recognized as seven days.|
|fortnight||none||2 weeks = 1 fortnight||A week is an old measure of time equalling two weeks. It is rarely used now.|
|month||mn||1 month = 28, 29, 30 or 31 days|
12 months = 1 year
|Months are human creations designed to break up time into manageable units between a day and a year. Currently, most people use the Gregorian calendar which sets the lengths of the months as follows: January 31, February 28 or 29, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, and December 31.|
|year||yr||1 year ≈ 365 1/4 days|
1 year = 12 months
10 years = 1 decade
100 years = 1 century
|A year is the time it takes the for the earth to rotate once around the sun. Since this can vary slightly because of the gravitational attraction of other planets, 1 year can not be expressed exactly in seconds. As a part of Coordinated Universal Time, scientist add and subtract seconds from various days in a year to keep clocks synchronized with the actual orbit of the earth.|
|decade||none||10 years = 1 decade10 decades = 1 century||While a decade can mean any 10 year period, the term is often used to indicate a 10 year period beginning on January 1 of a year whose number ends in zero, such as 2010.|
|century||none||100 years = 1 century|
10 centuries = 1 millennium
|While a century can mean any 100 year period, the term is often used to indicate a 100 year period beginning on January 1 of a year whose number ends in two zeros, such as 2000.|
|millennium||none||1000 years = 1 millennium|
10 centuries = 1 millennium
|While a millennium can mean any 1000 year period, the term is often used to indicate a 1000 year period beginning on January 1 of a year whose number ends in three zeros, such as 2000.|
|Table 1: Common units of measure of time|
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