Neither a year nor an annus can be a derived unit in the SI

Chemistry International



The year is not a unit of the SI. The only SI unit of measurement for time is the second. The word “annus” or “annum” does not appear anywhere in the current SI document. The word “year” is not in the table of “Non-SI units accepted for use with the International System of Units,” nor in the table of “Non-SI units whose values in SI units must be obtained experimentally,” nor even in the table of “Other non-SI units.” The year can be found, however, through the list of “Other non-SI units not recommended for use.” This heading directs the reader to a National Institute of Standards and Technology (NIST) list where three kinds of year (365 days, sidereal, and tropical) are given with conversion to seconds, but are set in type to indicate “in general not to be used in NIST publications.” Table 1 summarizes some of the uses of the year in other publications. For example, in the IUPAC chemistry document, the year is not a constant; in the International Astronomy Union Style Guide, the year (Julian) is a constant. 


Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Neither a year nor an annus can be a derived unit in the SI
Series title Chemistry International
Volume 33
Issue 6
Year Published 2011
Language English
Publisher International Union of Pure and Applied Chemistry
Contributing office(s) Eastern Geology and Paleoclimate Science Center
First page S1-S2
Online Only (Y/N) N
Additional Online Files (Y/N) N
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