Typical units for distance used in Astronomy:

1 AU = Astronomical Unit = average Earth-Sun distance

1 AU = 1.495979 x 10^{11} m = 1.495979 x 10^{8} km.

1 ly = Light Year = distance traveled by light in one year

1 ly = 9.46053 x 10^{15} m = 9.46053 x 10^{12} km = 0.30659486 pc.

1 pc = Parsec = distance at which a star has parallax angle of 1 arc-second.
( 206265 is the number of arc-seconds in one radian of angular measure.)

1 pc = 206265 AU = 3.085678 x 10^{16} m = 3.085678 x 10^{13} km = 3.261633 ly.

Of course, then we can multiply any these by one thousand or one million or one billion to get bigger units: kpc, Mpc, and Gpc, for example.

Last update: April 13, 1998.

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