Time

Time is the never-ending continued proсess of existence and events. It happens in an apparently irreversible way from the past, through the present and to the future.

To measure time, we can use anything that repeats itself regularly. One example is the start of a new day (as Earth rotates on its axis). Two more are the phases of the moon (as it orbits the Earth), and the seasons of the year (as the Earth orbits the Sun).

Even in ancient times, people developed calendars to keep track of the number of days in a year. They also developed sundials that used the moving shadows cast by the sun through the day to measure times smaller than a day. Today, highly accurate clocks can measure time in less than a billionth of a second. The study of time measurement is known as horology.

The SI (International Systems of Units) unit of time is one second, written as s. When used as a variable in mathematics, time is often represented by the symbol [math]\displaystyle{ t }[/math].

In Einsteinian physics, time and space can be combined into a single concept. For more on the topic, see space-time continuum.

Units of time

• 1 millennium = 10 centuries = 100 decades = 200 lustrums = 250 quadrenniums = 333.33 trienniums = 500 bienniums = 1,000 years
• 1 century = 10 decades = 20 lustrums = 25 quadrenniums = 33.33 trienniums = 50 bienniums = 100 years
• 1 decade = 2 lustrums = 2.5 quadrenniums = 3.33 trienniums = 5 bienniums = 10 years
• 1 year = 12 months = 52 weeks = 365 days (366 days in leap years)
• 1 month = 4 weeks = 2 fortnights = 28 to 31 days
• 1 fortnight = 2 weeks = 14 days
• 1 week = 7 days
• 1 day = 24 hours
• 1 hour = 60 minutes
• 1 minute = 60 seconds
• 1 second = SI base unit of time
• 1 millisecond = 1/1,000 second
• 1 microsecond = 1/1,000,000 second
• 1 nanosecond = 1/1,000,000,000 second
• 1 picosecond = 1/1,000,000,000,000 of a second
• 1 femtosecond = 1/1,000,000,000,000,000 of a second
• 1 attosecond = 1/1,000,000,000,000,000,000 of a second
• 1 Planck time = smallest measurable unit of time

Things used to measure time

• File:A digital alarm clock in Glasgow.jpg

  a digital clock in Glasgow
  Clock
• Hourglass
• Stopwatch
• Sundial
• Circadian rhythm

Time of day

• World Clock
• U.S. Naval Observatory Archived 2010-08-30 at the Wayback Machine
• Time in Archived 2011-08-10 at the Wayback Machine Hexadecimal numeral system

Time Media

• Wooden hourglass 3.jpg

  The flow of sand in an hourglass can be used to measure the passage of one hour of time. It also concretely represents the present as being between the past and the future.

• Horizontal sundial in Canberra

• 24-hour clock face in Florence

• Swatch Irony angle below.jpg

  A contemporary quartz watch, 2007

• ChipScaleClock2 HR.jpg

  Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.

• Jain scale of time.JPG

  Scale of time in Jain texts shown logarithmically

• Le Temps.JPG

  Time's mortal aspect is personified in this bronze statue by Charles van der Stappen.

• SVG version of World line.png

• Relativity of Simultaneity.svg

  Relativity of simultaneity: Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and occurs later in the red frame.

• Lorentz transform of world line.gif

  Lorentz transform of world line

Related pages

• Chronology
• Chronos
• 24-hour clock
• Time zone
• Space-time