Parsecs, etc.
John, KA9MNK
I guess we'll never know if Lucas just goofed or if this was one of his
(many) subtle jokes. I suspect the latter, but anyway...Someone brought
up the point that the parsec is not, by any means, a universal constant
as it is based on the length of the (major?) axis of the orbit of the
definer's home planet (in our case, Good Ol' Terra). This is so obviously
true once pointed out, that I was surprised that I had never considered it
before. Every time I would read (or hear) the term "parsec" I would just
automatically convert it to lightyears and say, "Yep, that's quite a distance,
all right," and not give it a second thought.
Another thing I just realized, along the same line, is that the lightyear isn't
universal, either. A Mercurian's lightyear would be a heck of a lot shorter
then a Plutonian's.
It's funny how these obviously parochial measures come to be considered
universal absolutes. I guess it's what the developmental psychologists call
"irreversibility." You just can't take the other {person|creature|???}'s
point of view.
May the Force be with you,
(And also with you),
John (I used to be Catholic but I outgrew it) Opalko
Gary Benson
Webster's says it is equal to a *heliocentric* paralllax of one arc
second. It is then converted to earth radii, then to light-years. Seems to
me that what a parsec is is a distance that you have to be away from a sun
to observe one arc-second of parallax. Maybe a scientist who really knows,
or someone who cares to research it more thoroughly would care to set us
all straight...
may the force be be with you
and with your spirit
Gary (also outgrew Rome) Benson
!fluke!inc
--
---
Gary Benson
John Fluke Mfg. Co.
Everett, WA, USA
P.C. Criqui
You apparently outgrew the notion that the speed of light is a constant at the
same time that you outgrew Catholocism. Perhaps you should study Physics more
religiously. You would discover that the speed of light is the same on
Mercury as it is on Pluto!
Pete Criqui
AT&T Technologies @ Bell Labs - Short Hills
John, KA9MNK
For John Opalko,
=========================================================================
For Pete Criqui,
No, the speed of light is definitely the same on Mercury as on Pluto (along
with everywhere else) but the *lengths of their years* sure aren't.
A Plutonian lightyear is one heck of a lot longer than a Mercurian LY.
THAT was the example I was trying to express.
Of course, unless there are sentient Mercurians and Plutonians trying to
create a system of measures, the whole point is academic anyway. :-)
John Opalko
AT&T Bell Labs (or whatever we're being called this week)
- Naperville, IL
Jonathan Biggar
>Are you *sure* that a parsec is a local unit of measurement? My big old
>Webster's says it is equal to a *heliocentric* paralllax of one arc
>second. It is then converted to earth radii, then to light-years. Seems to
>me that what a parsec is is a distance that you have to be away from a sun
>to observe one arc-second of parallax. Maybe a scientist who really knows,
>or someone who cares to research it more thoroughly would care to set us
>all straight...
If the position of a star when seen against the background sky changes
by one second of arc when measured from two points on the Earth's
orbit that are on the opposite sides of the sun from each other, then
the distance to that star is exactly 1 parsec.
Note that the Earth's orbit is a factor. Because Mars (for example) has
a larger orbit diameter, a star that is one Earth parsec away will
be measured as being less than one Mars parsec away.
A Mars parsec is greater than an Earth parsec by the same
proportion that Mars' orbit is greater than the Earth's.
Proof:
Mars' orbit
|-------------------------|
| |
| Earth's |
| orbit |
| |-----------| |
v v v v
A B O C D
Sun
Star
o
D' C' B' A'
(Measured locations of the star against the background sky)
Imagine lines between A and A', B and B' etc. (I can't draw them)
All of these lines intersect at o (the star).
1) angle D'oA' = angle AoD and angle C'oB' = BoC
(when two lines intersect, opposite angles are congruent.)
2) angle ADD' = angle DD'A', angle DAA' = angle AA'D',
angle BCC' = angle CC'B', angle CBB' = angle BB'C'
(opposite interior angles are congruent)
3) triangle ADo and A'D'o are similar, triangle BCo and B'C'o are similar
(angle, angle, angle)
4) therefore AD A'D'
------ = -------- => the measured parallax is
BC B'C' proportional to the orbit size
Therefore, for a planet whose orbit is twice the diameter of
Earth's, it's parsec is twice as long as Earth's.
Thus, the parsec is not a universal physical constant.
--
Jon Biggar
{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!jonab