Astronomy 123 Basics   Distance Scales 

There are two distance scales in use in Astronomy: 

The Light-year (ly): 
• the distance light will travel in one year
• 1 ly = 9.46 x 10¹² 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 10⁷)


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 light-years