蒙德極小期
Although the physical aspects of sunspots were not identified until the 20th century, observations continued. Study was hampered during the 17th century due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 19th century, then-sufficient sunspot records allowed researchers to infer periodic cycles in sunspot activity. In 1845, Henry and Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas. The emission of higher than average amounts of radiation later were observed from the solar faculae.