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RE: Re: LightYear Conversion is Incorrect in Units Package

All the figures I give here are the same in the NIST Guide to SI Units which
Urijah gives a link to, and the CRC Handbook of Chemistry and Physics 75th

The conversion for LightYear is 9.46073*^15 Meter.
(Mathematica uses 9.4605*^15 Meter)

The conversion for TropicalYear is 3.155693*^7 Second (== 365.2422454 Day)
The conversion for SiderealYear is 3.155815*^7 Second (== 365.2563657 Day,
close to Mathematica)
(Mathematica has 365.24219 Day for the TropicalYear)

I changed the Units package by putting in the better value for LightYear.
Then if I convert 1 LightYear to Seconds (using c = 1) I obtain
LightYear -> 3.15575984236401*^7 Second

This is just in between the TropicalYear and SideralYear. The NIST notes on
light year say: "This conversion is based on the astronomical unit of time
of one day (86400 seconds); an interval of 36525 days is one Julian
century." They don't tell us how many days are taken in a year. Why light
year is not linked to some common definition of Year is beyond me.

David Park
djmp at

> From: Urijah Kaplan [mailto:uak at]
To: mathgroup at
> The plot seems that Mathematica uses a year of
> 365.24219 days
> or so, but the National Institute of Science and Technology says (see
> page 64 of the pdf or you
> can find it at ) mandates
> that for this purpose, a year is 365.25 days, which makes a light year
> 9.4607*^15*Meter, which is different from what Mathematica says.
> So it is a
> bug!

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