An Etymological Dictionary of Astronomy and Astrophysics

An area seen as a dark patch on the Sun’s surface. Sunspots appear dark because they are cooler (of about 4000 °C) than the surrounding → photosphere (about 6000 °C). They range in size from a few hundred kilometers to several times the Earth’s diameter and last from a few hours to a few months. Very small sunspots are called → pores. The number of sunspots
varies from maximum to minimum in about 11 years, the
→ sunspot cycle. Their appearance during a cycle follows the → Sporer law. A typical spot has a central → umbra surrounded by a → penumbra, although either features can exist without the other. Sunspots are associated with strong magnetic fields of 0.2 to 0.4 → tesla. A given sunspot has a single magnetic → polarity. The opposite polarity may be found in other sunspots or in the bright and diffuse → facular region adjacent to the sunspot. The first recorded naked-eye sightings of sunspots were by Chinese astronomers in the first century B.C.
Johannes Fabricius (1587-1617) was the first to argue that sunspots are areas on the solar surface.

See also: → Sun; → spot.