RE: Re: LightYear Conversion is Incorrect in Units Package

• To: mathgroup at smc.vnet.net
• Subject: [mg28637] RE: [mg28627] Re: LightYear Conversion is Incorrect in Units Package
• From: "David Park" <djmp at earthlink.net>
• Date: Thu, 3 May 2001 04:28:27 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Investigating some more, it appears that the LightYear is the distance that
light travels in 365.25 days (what might be called a JulianYear?). Since the
speed of light is defined exactly, and the number of seconds in a Julian
year is also exact, there is no reason why the conversion from LightYear to
Meter can't be exact.

Needs["Miscellaneous`PhysicalConstants`"]

(365 + 1/4)(60 60 24)Second
%SpeedOfLight
31557600 Second
9460730472580800 Meter

For those who care about such things, the above conversion factor could be
substituted in the Unit package for 9.4605*^15 Meter (Mathematica) or
9.46073*^15 Meter (NIST and Handbook figure).

Also 1 LightYear is equivalent to 1461/1460 Year.

David Park

> From: Urijah Kaplan [mailto:uak at sas.upenn.edu]
To: mathgroup at smc.vnet.net
>
> The plot thickens...it seems that Mathematica uses a year of
> 365.24219 days
> or so, but the National Institute of Science and Technology says (see
> http://physics.nist.gov/Pubs/SP811/contents.html page 64 of the pdf or you
> can find it at http://physics.nist.gov/Pubs/SP811/appenB8.html ) 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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