Editing Time standard (section)

== Time standards based on Earth rotation ==
[[Apparent solar time]] or true solar time is based on the solar day, which is the period between one solar noon (passage of the real Sun across the [[meridian (geography)|meridian]]) and the next. A solar day is approximately 24 hours of mean time. Because the Earth's orbit around the Sun is elliptical, and because of the obliquity of the Earth's axis relative to the [[ecliptic|plane of the orbit (the ecliptic)]], the apparent solar day varies a few dozen seconds above or below the mean value of 24 hours. As the variation accumulates over a few weeks, there are differences as large as 16 minutes between apparent solar time and mean solar time (see [[Equation of time]]). However, these variations cancel out over a year. There are also other perturbations such as Earth's wobble, but these are less than a second per year.

[[Sidereal time]] is time by the stars. A sidereal rotation is the time it takes the Earth to make one revolution with rotation to the stars, approximately 23 hours 56 minutes 4 seconds. A mean solar day is about 3 minutes 56 seconds longer than a mean sidereal day, or {{fract|1|366}} more than a mean sidereal day. In [[astronomy]], sidereal time is used to predict when a [[star]] will reach its [[culmination|highest point]] in the sky. For accurate astronomical work on land, it was usual to observe sidereal time rather than solar time to measure mean solar time, because the observations of 'fixed' stars could be measured and reduced more accurately than observations of the Sun (in spite of the need to make various small compensations, for refraction, aberration, precession, nutation and proper motion).  It is well known that observations of the Sun pose substantial obstacles to the achievement of accuracy in measurement.<ref>See H A Harvey, [http://adsabs.harvard.edu/full/1936PA.....44..533H "The Simpler Aspects of Celestial Mechanics"], in Popular Astronomy 44 (1936), 533-541.</ref> In former times, before the distribution of accurate time signals, it was part of the routine work at any observatory to observe the sidereal times of meridian transit of selected 'clock stars' (of well-known position and movement), and to use these to correct observatory clocks running local mean sidereal time; but nowadays local sidereal time is usually generated by computer, based on time signals.<ref>A E Roy, D Clarke, [https://books.google.com/books?id=v2S6XV8dsIAC&pg=PA89 'Astronomy: Principles and Practice' (4th edition, 2003) at p.89].</ref>

{{Anchor|Mean solar time}}[[Mean solar time]] was a time standard used especially at sea for navigational purposes, calculated by observing apparent solar time and then adding to it a correction, the [[equation of time]], which compensated for two known irregularities in the length of the day, caused by the ellipticity of the Earth's orbit and the obliquity of the Earth's equator and polar axis to the [[ecliptic]] (which is the plane of the Earth's orbit around the sun). It has been superseded by [[Universal Time]].

[[Greenwich Mean Time]] was originally mean time deduced from meridian observations made at the [[Royal Greenwich Observatory]] (RGO). The principal meridian of that observatory was chosen in 1884 by the [[International Meridian Conference]] to be the [[Prime Meridian]].  GMT either by that name or as 'mean time at Greenwich' used to be an international time standard, but is no longer so; it was initially renamed in 1928 as Universal Time (UT) (partly as a result of ambiguities arising from the changed practice of starting the astronomical day at midnight instead of at noon, adopted as from 1 January 1925).  [[UT1]] is still in reality mean time at Greenwich. Today, GMT is a [[time zone]] but is still the legal time in the UK in winter (and as adjusted by one hour for summer time).  But [[Coordinated Universal Time]] (UTC) (an atomic-based time scale which is always kept within 0.9 second of UT1) is in common actual use in the UK, and the name GMT is often used to refer to it. (See articles [[Greenwich Mean Time]], [[Universal Time]], [[Coordinated Universal Time]] and the sources they cite.)

Versions of [[Universal Time]] such as UT0 and UT2 have been defined but are no longer in use.{{sfn|Urban|Seidelmann|2013|page=81}}<ref>{{cite web |last1=Schlyter |first1=Paul |title=Time Scales: UT1, UTC, TAI, ET, TT, GPS time |url=https://www.stjarnhimlen.se/comp/time.html |url-status=live |archive-url=https://web.archive.org/web/20250102032517/http://stjarnhimlen.se/comp/time.html |archive-date=2025-01-02 |access-date=2025-01-23 |website=www.stjarnhimlen.se |quote=UT2 is nowadays considered obsolete.}}</ref>