Parallax measurement of distance, illustration. Parallax is used to measure astronomical distances. Parallax is the shift of a nearby object against a distant background when viewed from two different positions. Here, a nearby star is viewed from either side of Earth's orbit, the largest possible baseline. Parallax measurements are given in parsecs, where an object with a 6-month parallax shift of 2 seconds of arc is at a distance of one parsec. One astronomical unit (AU, the Earth-Sun distance) is 150 million kilometres. A parsec is 30,900 trillion kilometres. Parallax can be detected for stars up to 100 parsecs away.