One of eight colour lithographs (Pl.5) with window mounts of six colour sketches showing sunspots in white light, drawn by Etienne Leopold Trouvelot in 1872. Successive views of a sun spot, March 28th, 12 20; March 30th 3 30; April 1st, ; April 2nd; April 3rd; April 4th;
Lithographic colour print issued by Harvard College Observatory in 1876, showing close-ups of the solar photosphere, the visible surface of the Sun. Based on sketches made Etienne Leopold Trouvelot (1827-1895), a French artist, they show the appearance sunspots in white light on six dates during 1872. These black markings with a dark (umbra) surrounded by a lighter region (penumbra) are areas of cooler gas on the Sun's surface. With a dazzlingly bright surface and a temperature of over six thousand degrees centigrade their darkness is a contrast effects. The number of sunspots slowly increases to a maximum and then falls back in a clear 11-year cycle first recognised by the German astronomer Heinrich Schwabe in 1843.