Langley's sunspot observation. Artwork showing an extended observation of a sunspot, made on 24 December 1873 by US astronomer and inventor Samuel Pierpont Langley (1834-1906). Sunspots, first observed with telescopes in 1610, are areas of magnetic activity that are cooler than the rest of the Sun's surface. They are massive structures, ranging in size from hundreds to thousands of kilometres across. This artwork shows the cooler central areas (dark) and the hotter (lighter) areas round the sunspot. This copy of Langley's drawing is from a German publication, and has a German caption across bottom.