Quote:
Originally Posted by tusenfem
I haven't got a clue how you come up with that equation above, the square root of "Right Ascention" squared plus "Declination" squared? At least to have a result in arcseconds per year you should have a Δ or something in there.
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Right ascension is movement across, and declination is movement up or down. If the heavens were a flat Euclidean sheet, the line of proper motion would be the hypotenuse of the triangle formed by the orthogonal lines of right ascension and declination, giving its length, the distance of proper motion, as the square root of the added squares of the other sides by geometry.
I assume it is the curvature of the celestial sphere that produces the 10% difference between the Pythagorean number and the actual result for Vega.