Astronomy Basics
Distance Scales
There are two distance scales in use in Astronomy:
The Lightyear (ly):

the distance light will travel in one year

1 ly = 9.46 x 10^{12} kilometers

you can calculate this distance by multiplying
 the
speed of light (300,000 kilometers/second)
 by the number of seconds in
a year (3.15 x 10^{7})

 NASA 
The parsec (psec)

related to the apparent motion of a close object compared
to the relatively more distant background over the course of one year (see
illustration
 Fig 17.1 of textbook)
 if the angle of parallax is 1 second of arc, the distance is 1 parsec

1 psec = 3.1 x 10 ^{13 }kilometers
1 parsec = 3.26 lightyears
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