Distances to the closest stars can be determined through measurement of their trigonometric parallax. The parsec was defined to be the distance at which 1 AU (perpendicular to the line of sight) subtends an angle of 1 arcsecond:

1 parsec (pc) = distance d when angle is 1 arcsecond = 3.086 10¹³ km = 3.26 ly

Using the Earth’s orbit as a baseline, the distance (in parsecs) to a star can be calculated using:

d = 1/p

where p is measured in arcseconds.