VOunits draft

[Steve Allen]

On Tue 2009-05-26T20:56:50 +0100, Anita M. S. Richards hath writ:
> I very much hope that we will do all of these - and Mpc and even Ypc....

Take a close look at the FITS WCS paper 1 table.
Along with all the true SI units it admits a number of astronomical
units, in particular "a" for "annum", year.

But there is an SI unit pascal, "Pa", which is thus indistinguishable
from Pa, for "peta annum".

As a result, the FITS WCS standard makes an explicit exception by
asserting that Pa may not mean "peta annum".

This is simply a "heads up" that to admit any units other than SI is
to open the door to various sorts of special cases which have to be
coded into all implementations of unit parsing code.

--
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