When looking at the radius of the revolution orbit of the earth against distant stars or, in other words, when looking at the earth and the sun, their angles look different (stellar parallax). When this difference is n-arcseconds (1/3600 of a degree), the distance between the sun and the star is 1/n parsec. It is 1 parsec for 1 arcsecond. It is equivalent to 3.262 light years.