I don't think we need an SI unit for year at all. The word itself is needed (and y would be a convenient abbreviation [which is not the same as a symbol]), as it would be difficult without it to talk about the passage of time in human terms, but the fact is that it isn't a precisely-defined unit of time (and, therefore, not an exact multiple of any of the approved non-SI terms, let alone the SI base unit, the second).
The largest unit for which one could justify approval (and a symbol) is week. However, it gets somewhat flakey once we get above the hour (h), given that the earth's rate of rotation changes (downwards) over time. Bill -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Pierre Abbat Sent: Friday, August 24, 2007 05:35 To: U.S. Metric Association Subject: [USMA:39275] RE: Energy units On Friday 24 August 2007 03:02, Bill Potts wrote: > There's a problem with y for year. > > The unit y is neither an SI unit nor a unit approved for use with SI. > The largest unit that is approved is d, for day. I've also seen "a" for year, often in "Ma". This conflicts with the are, but I think that the are should be totally gotten rid of. Neither it nor its commonly used multiple, the hectare, agrees with the principle of thousands. If we should have a symbol for the year, which year should it be? There are the tropical year, the sidereal year, and several calendar years. Pierre
