In 1769 a Scottish astronomer named Alexander Wilson, working at the Macfarlane Observatory

Macfarlane Observatory

At Glasgow University, the Macfarlane Observatory was established in 1757 with instruments donated by Alexander Macfarlane, a merchant in Jamaica. The instruments arrived to Glasgow in a deteriorated condition, and their suitability for mounting was in question before they were taken in hand by...

, noticed that the shape of sunspot

Sunspot

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

s noticeably flattened as they approached the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's limb due to the solar rotation

Solar rotation

Solar rotation is able to vary with latitude because the Sun is composed of a gaseous plasma. The rate of rotation is observed to be fastest at the equator , and to decrease as latitude increases...

. These observations showed that sunspots were features on the solar surface, as opposed to minor planets or objects above it. Moreover, he observed what is now termed the Wilson effect: the penumbra and umbra

Umbra

The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....

 vary in the manner expected by perspective effects if the umbrae of the spots are in fact slight depressions in the surface of the photosphere

Photosphere

The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

. The magnitude of the depression is difficult to determine but may be as large as 1,000 km.

While the surface-depression interpretation of the Wilson effect is widespread, Bray and Loughhead (1965) contended that "the true explanation of the Wilson effect lies in the higher transparency of the spot material compared to the photosphere". They develop this idea on pages 93 to 99 of their book. A similar interpretation was expressed by C.H. Tong in 2005.

Sunspots result from the blockage of convective heat transport by intense magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

s. Sunspots are cooler than the rest of the photosphere, with effective temperatures of about 4000°C (about 7000°F). Sunspot occurrence follows an approximately 11-year period known as the solar cycle

Solar cycle

The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...

, discovered by Heinrich Schwabe

Heinrich Schwabe

Samuel Heinrich Schwabe a German astronomer remembered for his work on sunspots.Schwabe was born at Dessau. At first an apothecary, he turned his attention to astronomy, and in 1826 commenced his observations on sunspots. Schwabe was trying to discover a new planet inside the orbit of Mercury...

in the 19th century.