Parnips, er, parsecs

[Scott Fisher <sefisher@cisco.com>]

Okay, here's a little reminder about what parallax is.  You stand here
and look just past that tree (no, not that one, the big one over there),
you see a cow out in the field.  You move a couple feet to your left and
the cow is now hidden behind the tree.  That's parallax.

Now, let's apply this to space.  Today is Sept. 1.  You look in the sky
this evening and you see a star and, because you're an astronomer (or at
least playing one on the Net), you mark its location very, very
carefully.  You wait six months; it's March 1, the Earth is 180 degrees
away from its current location, you look and you see that same star,
only it's now moved one second of arc (that is, 1/60th of a minute,
which is 1/60th of a degree, for a total displacement of 1/3600th of a
degree) away from the place it was on Sept. 1.

So how far away does an object have to be for its parallax to equal one
second of arc?

One parsec -- one SECond of arc when viewed at two locations defined by
the PARallax of the Earth's rotation around the Sun.

This works out to 3.25 light years (though I could have SWORN that I'd
learned it as 4.3 light years; ah well, the Earth has slowed down since
I were a lad, it's probably got a shorter orbit these days too... "Ah,
no, young Padewan, it's the distance from Earth to Alpha Centauri which
is 4.3 light years, which works out to 1.33 parsecs.").

So the space geek response to Lucas' joke about "She's the ship that
made the Kessel run in less than 12 parsecs" is that Han Solo is using
the parsec as a measure of time (duh, well, it's 3.25 of some kinda
year, innit?)  This falls into the same category as Lucas' sound effects
for explosions in space, which only exist according the the Looney Tunes
Laws of Physics ("But where was the kaboom?  There was supposed to be an
earth-shattering kaboom!")

And it will surprise no one, I'm sure, when one works out the number of
light years equivalent to 12 parsecs.  What else COULD it be?

To bring this topic around to something more appropriate to this list, I
propose the following:

You know how people in the passenger's seat always seem to think you're
going faster than you do in the driver's seat, even though the speed is
the same?  Well, I propose that the relative difference between
perceived speed when the observer shifts from the left-hand seat to the
right (or in Phil's case, from right to left) be referred to as "the
carsec."

As in "Honeeeeeeeeeeeey, slow DOWN, you're making me carsec!"

--Scott "Rocket science at short notice was his specialty" Fisher