Sunspot. Drawing by Samuel Langley (1834-1906), solar physicist, of a sunspot. Sunspots are dark areas on the Sun's surface caused by fluctuating magnetic fields within the Sun. They typically have a dark centre ( Iumbra i) and a less dark Ipenumbra i, as drawn here. Sunspot numbers vary in cycles of around 11 years, the cause of which is not fully understood. This drawing was made on 23 December 1873, during a period of high sunspot activity. Langley was a self-taught astronomer, and was also a pioneer of aircraft design. He invented the bolometer in 1880, which measures the infra-red energy of the Sun. This effectively started the field of infra-red astronomy.