Time

Pronunciation: /taɪm/ Explain

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 is 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.

Common Units of Measure of Time

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 day, 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 fortnight 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 decade 10 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

References

1. McAdams, David E.. All Math Words Dictionary, time. 2nd Classroom edition 20150108-4799968. pg 178. Life is a Story Problem LLC. January 8, 2015. Buy the book

Cite this article as:

McAdams, David E. Time. 4/3/2019. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/t/time.html.

Revision History

12/21/2018: Reviewed and corrected IPA pronunication. (McAdams, David E.)
12/4/2018: Removed broken links, updated license, implemented new markup. (McAdams, David E.)
5/5/2011: Initial version. (McAdams, David E.)