inc@fluke.UUCP (Gary Benson) (12/19/83)
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... 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
jonab@sdcrdcf.UUCP (Jonathan Biggar) (12/21/83)
In article <138@tpvax.fluke.UUCP> inc@fluke.UUCP (Gary Benson) writes: >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
rigney@uokvax.UUCP (12/28/83)
#R:tpvax:-13800:uokvax:12300011:000:1151 uokvax!rigney Dec 26 11:58:00 1983 No, a parsec is the distance from which a star observed from opposite sides of Earth's orbit is seen to shift one arc-second of parallax. Not only does it depend on Earth's orbit, but it also depends on the measurement of an an arc-second, which is arbitrarily (from a universal point of view) set at 1/1296000 of a complete circle. Poorly phrased, but you get the idea? You could use a parrad, for a parallax of one radian (A radian is a universal measurement, pi is pi everywhere), except this still depends on the radius of the Earth's orbit. And you can't use any other radius either, because there's no universal radius. I say can't, but obviously you can use such a measurement. I suspect in a future galactic standard there would be many differing local standards of measurement, and just a few universal standards, based on powers of 2, times the fundamental constants. The only difference in universal standards, of course, would be scaling factors and names; that's why they're called universal. Anyone care to post suggestions? Do we care:-? Carl ..!ctvax!uokvax!rigney