Path: utzoo!attcan!uunet!husc6!bloom-beacon!gatech!rutgers!njin!princeton!phoenix!pupthy!wrs
From: wrs@pupthy.PRINCETON.EDU (William R. Somsky)
Newsgroups: comp.text
Subject: Re: Definition of Point
Message-ID: <3965@phoenix.Princeton.EDU>
Date: 13 Oct 88 21:29:17 GMT
References: <2610015@hpsal2.HP.COM> <142@ur-cc.UUCP>
Sender: news@phoenix.Princeton.EDU
Reply-To: wrs@pupthy.PRINCETON.EDU (William R. Somsky)
Organization: Physics Dept, Princeton Univ
Lines: 42

In reply to Michael Morrell <2610015@hpsal2.HP.COM> morrell@hpsal2.HP.COM
>> Does anyone know the exact definition of "point" (as in a 12-point font)?

Roland Roberts <142@ur-cc.UUCP> rbr4@uhura.cc.rochester.edu writes:
> ... the following are from the TeXbook, p57.
> ...
>	1 inch = 72.27 point
> ...

If you continue reading, you find on page 58 of the TeXbook:

    [Dangerous]   The units have been defined here so that precise
    [  Bend   ]   conversion to sp is efficient on a wide variety
		  of machines. In order to achieve this, TeX's "pt"
    has been made slightly larger than the official printer's point,
    which was defined to equal exactly .013837in by the American
    Typefounders Association in 1886 [cf. National Bureau of Standards
    Circular 570 (1956)]. ...

Hence, if the American Typefounders Association definition
is still the official one, we have:

DEFINED (1886):  1 point =  .013837 in
DEFINED (19??):  1 in    = 2.54 cm
DEFINED (1983):  1 m     = (speed of light in vacuum) x 1/299792458 s
DEFINED (1977):  1 s     = "9192631770 periods of the radiation corresponding
                           to the transition between the two hyperfine levels
			   of the ground state of the cesium-133 atom"

So, if all my information is up to date,
just grab youself a cesium-133 atom,
get it to kick out a photon from its hyperfine transition in the ground state,
count off (.013837)(.0254)(9192631770)/(299792458) wavelengths in a vacuum,
and you'll have one point EXACTLY!

However, if you're not an overly pedantic physicist,
just take (1/72)in or (1/72.27)in or anything else about the same,
and it'll probably be close enough for all practical purposes. :-)

------------------------------------------------------------------------
William R. Somsky                          Physics Dept ; Princeton Univ
wrs@pupthy.Princeton.EDU                 PO Box 708 ; Princeton NJ 08544