An Etymological Dictionary of Astronomy and Astrophysics

A system for counting rotations of the Sun based on the mean → synodic rotation period of the Sun. Initially, Lord Carrington determined the solar rotation rate by watching low-latitude → sunspots. He defined a fixed solar coordinate system that rotates in a sidereal frame exactly once every 25.38 days. This means that the solar rotation period, as viewed from the Earth, is assumed to be constant. However, the synodic rotation rate varies during the year because of the changing speed of the Earth in its orbit and the mean synodic period is about 27.2753 days. Carrington rotation number 1 began on November 9, 1853.

See also: Named for Richard C. Harrington (1826-1875), British astronomer, who initiated the system; → rotation.