The arcsecond is also often used to describe small astronomical angles such as the angular diameters of planets (e.g. the angular diameter of Venus which varies between 10" and 60"), the proper motion of stars, the separation of components of binary star systems, and parallax, the small change of position of a star in the course of a year or of a solar system body as the Earth rotates. These small angles may also be written in milliarcseconds (mas), or thousandths of an arcsecond. The unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements.