Parallax Determining distances through parallax You don't need to know trigonometry to follow the discussion below. But if you do know trigonometry, let me point out that all of the angles used are very small, and I use approximations valid for small angles.
Astronomers like to measure angles in seconds of arc.
Then a convenient unit of distance is the unit that makes the constant in this equation equal to 1. This unit is called the parsec . One also hears astronomical distances expressed in light-years. A light year is how far light goes in one year.