What is Time?
'''Time''' is the never-ending continued [[sequence|proсess]] of [[existence]] and [[event]]s. 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]] [[rotation|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 [[calendar]]s to keep track of the number of days in a year. They also developed [[sundial]]s that used the moving shadows cast by the sun through the day to measure times smaller than a day. Today, highly accurate [[clock]]s 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'''.<ref>{{Cite web|title=Current definitions of the SI units|url=https://physics.nist.gov/cuu/Units/current.html#:~:text=The%20second,%20symbol%20s,%20is,is%20equal%20to%20s-1.|access-date=2020-08-16|website=physics.nist.gov|archive-date=2020-09-20|archive-url=https://web.archive.org/web/20200920012307/https://www.physics.nist.gov/cuu/Units/current.html#:~:text=The%20second,%20symbol%20s,%20is,is%20equal%20to%20s-1.|url-status=live}}</ref> When used as a variable in mathematics, time is often represented by the symbol <math>t</math>.<ref>{{Cite web|date=2020-03-01|title=Compendium of Mathematical Symbols|url=https://mathvault.ca/hub/higher-math/math-symbols/|access-date=2020-08-16|website=Math Vault|language=en-US|archive-date=2020-04-28|archive-url=https://web.archive.org/web/20200428101652/https://mathvault.ca/hub/higher-math/math-symbols/|url-status=live}}</ref> In [[Einstein]]ian 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