One of two heliographic coordinate systems used for identifying the position of features on the Sun’s surface. In the Carrington system, lines of longitude rotate with the Sun. A prime meridian, analogous to the Greenwich meridian on Earth, was defined to coincide with the central meridian of the Sun (as seen from Earth) at a specific time on 1853 November 9 when R. C. Carrington began his observations. A new Carrington rotation begins each time the prime meridian crosses the central meridian. The rotation period of the coordinate system varies throughout the year because of the changing distance of the Earth from the Sun, with an average value of 27.2753 days (the mean synodic period). This corresponds to a sidereal rotation period of 25.38 days. The Carrington longitude of the central meridian is 360° at the beginning of the Carrington rotation, and decreases to zero at the end. The longitude of a feature on the Sun (such as a sunspot) remains approximately constant in the Carrington system, in contrast to the Stonyhurst system.