buck%NRL-CSS@sri-unix.UUCP (12/29/83)
From: Joe Buck <buck@NRL-CSS>
There appears to be some confusion over the definition of "parsec".
"Parsec" comes from "parellax second". A nearby star appears to shift its
position with respect to the more distant stars when viewed six months
apart, because the earth has moved by 2 A.U.'s (about 186 million miles).
A star one parsec away appears to move one second of arc (1/3600 of a
degree) with respect to more distant stars.
* - star
/|\
/ | \ (NOT TO SCALE - obviously)
/ | \
/ | \
earth(JUN)o----*----o - earth(DEC)
sun
If the angle at the star (between the hypotenuse and the vertical)
is x, the distance from the earth to the star is 1 A.U. divided by sin x.
If x is small enough, sin x is roughly equal to x, if x is measured
in radians. So one parsec is
1 A.U. * 360 * 3600
-------------------
2 pi
which is about 1.9E13 miles, or a little over 3 light years.
As you can see, the parsec depends on the distance from the earth
to the sun, so it's doubtful someone in another galaxy would want
to use it. Also, there is no unit of time anywhere (as opposed to
the light-year), so I don't understand how this error arose.
Joe Buck