Parsec

A parsec is the main unit of distance for astronomers when talking about distances to stars and galaxies.

When we are looking at a star from Earth, we can view it twice a year, six months apart. When comparing the position of the star between those two months in respect to the background stars, we can measure what is called a parallax angle. When the parallax angle is one arcsec, the star's distance is one parsec.

Now, there are no stars within 1 parsec of Earth (one AU is much much less than one parsec, so the distance from the Sun to the star is approximately the same from the Earth to the star. If you replace the parallax angle with p and the distance to the star d, you can calculate the distance if you measure the parallax angle.

d = 1/p as long as d is measured in parsecs (pc) and p is measures in arcsec (")