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Pastimes : SI Grammar and Spelling Lab
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To: The Philosopher who wrote (2093)3/3/1999 6:18:00 PM
From: Jacques Chitte  Read Replies (2) of 4711
 
You're right on it with the latter option. The fixed sky is an astrometric convention. While the galaxy is slowly stirring like popcorn in a flushing toilet - and the entire universe is flexing and dilating like a poorly-drawn weather balloon - the distant stars are fixed on a usable human time scale. (Analogy: a grassy lawn is a blurred riot in time-lapse, but if you stare at it nothing at all is happening. But the snail is visibly moving against the fixed grassy backdrop. And the centipede is hauling multiple ass.) Against this effectively stable background, it is possible to plot and compute speed and angulat direction of a star's lateral motion. Furthermore the annual oscillation of the nearer stars is an artifact of parallax. The importance of parallax in establishing stellar distance scales cannot be overstressed. Without it, we would have been a century delayed in having a cosmic yardstick. The cosmic yardstick is the very backbone of modern galactic astronomy.

I first learned about parallax when I was six and my dad demonstrated his multimeter (the electric test thingy with a pair of ire probes and a sector dial). There was a mirrored strip behind the dial. I asked what that was and he said "a parallax strip". The trick is to line up the needle with its reflection. When the reflection is occluded, you are sure not to get a skewed reading from parallax. Since the needle and the scale don't share a plane, parallax can introduce quite significant measurement error.
Corollary: when your better half is driving and you see the speedo and it looks like she's going right on the speed limit - she's actually speeding. Parallax gives you an indicated value several mph on the low side. (Not valid for right-hand drive!)
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