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Vol. 33 No. 4
July-August 2011

Making an imPACt | Recent IUPAC technical reports and recommendations that affect the many fields of pure and applied chemistry.
See also www.iupac.org/publications/pac

IUPAC-IUGS Common Definition and Convention on the Use of the Year as a Derived Unit of Time (IUPAC Recommendations 2011)
Norman E. Holden, et al.
Pure and Applied Chemistry, 2011
Vol. 83, No. 5, pp. 1159–1162

The units of time (both absolute time and duration) most practical to use when dealing with very long times (e.g., in nuclear chemistry and earth and planetary sciences) are multiples of the year, or annus (a). The definition of the year in terms of the SI base unit, is no trivial matter, as the year is not commensurable with the day, and is not a constant. In view, however, of the necessity to define units for time in such a way that they can be considered as constant for practical purposes, it is here recommended to define the year directly on the basis of the SI unit, the second. Taking into account the non-relativistic estimate of astronomical decrease by 0.530 s per century, for the epoch 2000.0, 1 a = 31556925.445 s. Adoption of this definition, and abandonment of the use of distinct units for time differences, will bring the earth and planetary sciences into compliance with quantity calculus for SI and non-SI units of time.

http://dx.doi.org/10.1351/PAC-REC-09-01-22


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