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Pastimes : SI Grammar and Spelling Lab -- Ignore unavailable to you. Want to Upgrade?

To: Ilaine who wrote (2089)	3/3/1999 5:50:00 PM
From: Jacques Chitte	Read Replies (3) \| Respond to of 4711

Parallax is the principle behind rangefinders. A pair of optical tubes is held a fixed distance apart, and the angle between the two when they are zeroed on the object in question allows the range to be figured. Have you ever watched a squirrel jump? It waves its head around while staring at the proposed landing site. it's taking a parallax range reading better than the one available by a fixed binocular reading. Cats have a good enough parallax engine in their heads that they don't need to do the head thing. Astronomers figured out a neat way to use parallax on a grand scale. The earth's orbit is of known diameter. The nearer stars actually wobble against the "fixed" stellar background as the earth traces out its annual circuit. The degree of motion is a function of the earth's motion (fixed and known) and the star's distance. While this wobble is much too small to be detectable to the casual observer, astrometers (the folks who obviously had nothing better to do) succeeded in reading parallax down to the thousandth of an arc second. The word parsec, denoting a standard measure of distance on an interstellar scale, is a contraction of "parallax arcsecond". Oddly, parallax gives read of inverse distance - something with twice the parallax is twice as close. Nonetheless a parsec ( about two point six light years) is a measurement of straight distance. A star ten parsecs distant will move 0.100 arc second against the fixed sky.