Solar rotation

As the Sun rotates about its axis, the effects of different structures on the solar surface becomes apparent. Radiation from sunspots and faculae is not emitted uniformly in all directions and thus their position on the visible disk of the Sun directly influences the radiation that is measured at the Earth. For example, sunspots have maximum impact when they are at the center and faculae effects are maximized at about 70 from the central meridian (i.e., when they are near the edge of the visible disk).