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Old Feb 15th 2017, 10:34 AM   #1
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Physics Parallax method

Does anybody know why the distance increases as the parallax angle decreases ? I understand that they areinversely proportional but whats the exact science behind it ?
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Old Feb 15th 2017, 01:46 PM   #2
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Consider the baseline for the parallax measurement - call it's length W. Now construct a line perpendicular to the center of the baseline to the object whose distance is being measured, and call that length D. If we define the parallax angle as the angle between the line of sight from either end of the baseline to the object and a line perpendicular to the baseline, then from trigonometry the distance D is:

$\displaystyle \tan \theta = \frac {(W/2)} D$

and hence $\displaystyle D = \frac {(W/2)}{\tan \theta}$

Hence for a constant value of W decreasing $\displaystyle \theta$ means increasing D.
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Old Feb 15th 2017, 04:51 PM   #3
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Surely you have seen picture of angles? A basic property of all angles (other than those of measure 0) is that the two rays separate!

Last edited by HallsofIvy; Feb 17th 2017 at 03:29 AM.
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Old Feb 18th 2017, 02:15 AM   #4
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Does anybody know other methods to find the parallax instead of closing on eye experiment ? Like any other lab experiments that i could carry out
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Old Feb 18th 2017, 05:23 AM   #5
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Sure, it's easy to concoct an experiment using a meter stick, some string to use as sight lines, some tape, and a protractor to measure the distance from the center of the meter stick to some fixed point in the room that is perhaps about 3 meters away. Go ahead and try it, and let us know how it works out.
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Last edited by ChipB; Feb 19th 2017 at 07:20 AM.
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Old Feb 18th 2017, 05:34 PM   #6
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A standard method of determining the distance to a (relatively nearby) star is to photograph a start at times half a year apart (so the earth is at opposite ends of its orbit- as far apart as two positions of the earth can be) and compare its positions on a photographic plate to "fixed stars" (those that are far enough away that the don't appear to move relative to each other between two such times). The change in position give the angle by "parallax".
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