"BarryMargolin"@MIT-MULTICS.ARPA (01/04/84)
The definition of a parsec is the length of the perpendicular or side (I
don't remember which, but they are pretty close to each other) of an
isosoles (sp?) triangle whose base is 2 AU and whose vertex angle is 1
second. The relationship to the physical derivation of the term is that
a stationary (relative to the sun) object is one parsec away if it has a
parallax of one second of arc when viewed from opposite points in the
earth's orbit. This has probably been long abandoned as an accurate
definition, since the earth's orbit is not a circle, and therefore does
not have a constant diameter; it is presumably defined in terms of
light-years these days.
barmar