Colour prints of the visible surface of the Sun

One of eight colour lithographs (Pl.6) with window mounts of four colour sketches showing the Sun in white light, drawn by Etienne Leopold Trouvelot in 1871. From photographs taken with a corrected lens of 35 feet focus; Figure 1, October 5th, 12h 00m M; October 9th, 11h 45m A.M.; October 11th, 12h 15m P.M.; October 13th, 10h 50m, A.M.

Lithographic colour print issued by Harvard College Observatory in 1876, showing the solar photosphere, the visible surface of the Sun. Based on sketches made Etienne Leopold Trouvelot (1827-1895), a French artist, they show the Sun's appearance in white light on four dates during 1871. Most noticeable are dark markings called sunspots, which are areas of cooler gas on the Sun's surface. In marked contrast are white features called faculae, areas of solar surface that are hotter than their surroundings. With a dazzlingly bright surface and a temperature of over six thousand degrees centigrade these dark and light features are 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.