An Etymological Dictionary of Astronomy and Astrophysics
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Same as → Wolf number and → sunspot number.

→ relative; → sunspot; → number.

An average of 13 monthly → sunspot numbers, centered on the month of concern. The 1st and 13th months are given a weight of 0.5.

→ smooth; → sunspot; → number.

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.

→ Sun; → spot.

→ solar cycle.

→ sunspot; → cycle.

Periods of time when the → relative sunspot number is low. These periods of time occur approximately every 11 years and represent the minimum in the → sunspot cycle.

→ sunspot; → minimum.

A quantity which gives the number of sunspots at a given time. It is defined by the relationship R = k(10g + f), where R is the sunspot number, k is a constant depending on the observation conditions and the instrument used, g is the number of the groups and f is the number of individual spots that can be counted. Also called the → Wolf number and → relative sunspot number.

→ sunspot; → number.