The latitude and longitude of a feature on the Sun’s surface. Two systems of heliographic coordinates are in use: Carrington heliographic coordinates and Stonyhurst heliographic coordinates. They differ in terms of whether the lines of longitude are fixed relative to the Sun’s rotation or fixed relative to the observer. In either case it is necessary to know the position angle of the solar rotation axis (P) and the latitude (B₀) and longitude (L₀) of the centre of the solar disk at the time of observation; these are given in almanacs.