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Vol. 33 No. 6
November-December 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

Letters About the IUPAC-IUGS Common Definition and Convention on the Use of the Year as a Derived Unit of Time

A set of recommendations jointly prepared by IUPAC and the International Union of Geological Sciences (IUGS) was recently published in Pure and Applied Chemistry (2011, Vol. 83, No. 5, pp. 1159–1162), which triggered a couple of letters to Chemistry International. The recommendations abstract reads as follows:
The units of time (both absolute time and duration) most practical to use when dealing with very long times, for example, in nuclear chemistry and earth and planetary sciences, are multiples of the year, or annus (a). Its proposed definition in terms of the SI base unit for time, the second (s), for the epoch 2000.0 is 1 a = 3.1556925445 × 10
7 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.

In one letter, L.E. Edwards argues that neither a year nor an annus can be a derived unit in the SI. Instead, she states, SI derived units are products of powers of base units. In a separate letter, N. Christie-Blick argues that what is at stake is whether or not a necessary distinction exists between geohistorical dates and unconstrained spans of geological time.

Both letters, in full, and the invited reply by the IUPAC/IUGS task group are online as a supplement to this issue. While responding, the task group stated that “What a scientific convention can do is point to what is considered correct as a result of very long and very careful evaluations of all possible arguments that are available at a given time after weighing of their merits.”

www.iupac.org/publications/ci/2011/3306/pac_letters-sup.pdf


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