A parallax is the angle through which an object appears to move when viewed from different positions. In 1838, the Prussian astronomer Friedrich Wilhelm Bessel (1784–1846) used telescopes at the Königsberg Observatory to measure the parallax of the star 61 Cygni to be approximately 0.314 arc seconds, where one arc second is 36001 of a degree. He did this by comparing the apparent positions of 61 Cygni during the year, relative to a fixed backdrop of distant stars. 1 parsec (or “parallactic second”) is the distance to a star with a parallax of 1 arc second measured across the Earth's orbit. 1 parsec ≈3.26 light-years.