Re: [office-formula] Straw poll - slug (and lightyear)

["David A. Wheeler" <dwheeler@dwheeler.com>]

Patrick Durusau <patrick@durusau.net>:
> Conversion factor? Ok, I had to get up out of my chair but:
> light year ly = 9.460530 X 10^17 cm
> Source: Kenneth R. Lang, Astrophysical Data, Springer-Verlag, 1992.

No, that's inexact. For specification purposes, we should state an exact value if an exact value exists.

The speed of light in a vacuum is defined to be exactly 299 792 458 metres per second
(the meter is defined in terms of the speed of light in vacuum).
Source: International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 112, ISBN 92-822-2213-6,
http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
A year, for light-year purposes, is exactly 365.25 days, as we've already discussed.

Thus, a light-year (ly) is exactly =
(299792458 m/s)(60*60*24 s/day)(365.25 day/year)(1 year) meters.

I recommend that we state it this way, including a note that this is exact.  That way, any reader
will know why it is exactly that value, including doing unit-checking.
Better that than an inexact "magic number" that cannot be checked.

--- David A. Wheeler