Editing Year (section)

=== Julian year ===
{{Main|Julian year (astronomy)}}
The Julian year, as used in astronomy and other sciences, is a time unit now defined as exactly 365.25 days of {{val|86400}} [[Second|SI seconds]] each<ref name="ESttAA">{{cite book |author=<!-- not stated --> |date=1992 |title=Explanatory supplement to the Astronomical Almanac |url=https://archive.org/details/explanatorysuppl0003unse/page/698/mode/2up |location=Mills Valley, California |editor-last1=Seidelmann |editor-first1=P. Kenneth |publisher=University Science Books |page=698 |isbn=0-935702-68-7 |access-date=11 April 2025}}</ref> ("[[ephemeris day]]s"). This is one meaning of the unit "year" used in various scientific contexts. The Julian century of {{val|36525}} ephemeris days and the Julian millennium of {{val|365250}} ephemeris days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify an amount of time (not how many "real" years), for long time intervals where stating the number of ephemeris days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a [[light-year]].

In the [[Unified Code for Units of Measure]] (but not according to the [[International Union of Pure and Applied Physics]] or the [[International Union of Geological Sciences]], see below), the symbol 'a' (without subscript) always refers to the Julian year, 'a<sub>j</sub>', of exactly {{val|31557600}} [[second]]s.

: 365.25 d × {{val|86400|u=s}} = 1&nbsp;a = 1&nbsp;a<sub>j</sub> = {{val|31.5576}} [[Second#SI multiples|Ms]]

The [[#SI prefix multipliers|SI multiplier prefixes]] may be applied to it to form "ka", "Ma", etc.<ref name="UCUM" />

The scientific Julian year is not to be confused with a year in the Jullian calendar. The scientific Julian year is a multiple of the SI second; it is today “astronomical” only in the sense “used in astronomy”, whilst true astronomical years are based on the movements of celestial bodies.