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motion: TOC for Knowledge Concepts, Exercises, and Solutions

One useful mathematical relation to remember is the inverse relationship between 
distance and parallactic angle:
 ????????(angle in arcseconds) =  ---------------------
                             (distance in parsecs)

So, for example, if the parallax of a distant object, measured using Earth's orbit 
as a baseline:

     is 1 arcsecond, then the distance is  1 parsec,
     is 2 arcseconds,   "           "      1/2 parsec,
     is 3 arcseconds,   "           "      1/3 parsec,
     is 1/2 arcsecond,  "           "      2 parsecs,
     is 1/3 arcsecond,  "           "      3 parsecs,
     is 1/9 arcsecond,  "           "      9 parsecs,

and so on. This relationship is merely a coincidence of how a parsec is defined.