Jc3s5h added a comment.
I stopped reading after the first paragraph of the "Description" because it is false. I quote it here, in case it is edited, so my comments will remain clear. > The annum unit is used in Wikidata statements to represent half life of isotopes, this is shown by this query. 1 annum = 1 gregorian mean year = 31 556 926 s The value is also cited by the NASA in this document as used currently for calendar calculation by them. Based on comments at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team#%C2%AB_annum_%C2%BB_conversion_? I surmise that 31 556 926 s is based on this paper <http://doc.rero.ch/record/303694/files/pac-rec-09-01-22.pdf> by Holden et al. In that paper the length of the tropical year near AD 2000 is asserted to be 31 556 925.445 SI seconds. But the mean Gregorian year is not the same as the tropical year. As the NASA document (although intended for K-12 education) correctly states, the mean length of a Gregorian calendar day is 365.2425 **calendar days**. If we define a second to be 1/86400 calendar day, this is equal to 31,556,952 seconds. The length of these days has been measured in various ways, initially by simply observing the rising and setting of the Sun; later with astronomical observations of the Sun and stars to calibrate mechanical clocks in observatories. Today, atomic clocks are used but leap seconds are introduced at irregular, unpredictable intervals to keep calendar days in alignment with the observed rotation of the Earth on it's axis. So no precise SI equivalent can be given; it all depends on what period of time you decide to average over, and how much the rotation of the Earth on it's axis disagreed with atomic clocks during the averaging period. TASK DETAIL https://phabricator.wikimedia.org/T247027 EMAIL PREFERENCES https://phabricator.wikimedia.org/settings/panel/emailpreferences/ To: Jc3s5h Cc: Jc3s5h, Ladsgroup, Aklapper, Liuxinyu970226, Lydia_Pintscher, Lea_Lacroix_WMDE, TomT0m, darthmon_wmde, Nandana, Lahi, Gq86, Lucas_Werkmeister_WMDE, GoranSMilovanovic, QZanden, EBjune, merbst, LawExplorer, _jensen, rosalieper, Scott_WUaS, Jonas, Xmlizer, jkroll, Smalyshev, Wikidata-bugs, Jdouglas, aude, Tobias1984, Manybubbles, Mbch331
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