In astronomy, the Wilson effect is the perceived depression of a

sunspot Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...

umbra The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object of lesser size. In cases of equal or smaller impinging objects, only an umbra and penumba are generated. As ...

, or center, in the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

. The magnitude of the depression for the umbra is between 500 and 1000 km, with an average of 600 km. Sunspots result from the blockage of

convective Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

heat transport by intense

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

s. Sunspots are cooler than the rest of the photosphere, with effective temperatures of about 4,000°C (about 7,000°F). Sunspot occurrence follows an approximately 11-year period known as the

solar cycle The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of Modern Maximum, variations in the number of observed sunspots on the Sun ...

, discovered by

Heinrich Schwabe Samuel Heinrich Schwabe (25 October 1789 – 11 April 1875) was a German amateur astronomer remembered for his work on sunspots. He observed sunspots and made drawings of them from 1825 to 1867 and suggested in 1838 that there may be a ten-year cy ...

in the 19th century.

History

In 1769, during solar cycle 2, Scottish astronomer Alexander Wilson, working at the

Macfarlane Observatory The Macfarlane Observatory was established at the University of Glasgow in 1757. It was the first purpose-built university observatory in Britain. History The Observatory was named after Alexander MacFarlane, a slaveholding Scottish merchant ...

, noticed that the shape of

s noticeably flattened as they approached the Sun's

limb Limb may refer to: Science and technology *Limb (anatomy), an appendage of a human or animal *Limb, a large or main branch of a tree *Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb *Limb, in botany, t ...

due to

solar rotation Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy ...

. R.J. Bray and R.E. Loughhead (1965) ''Sunspots'', page 4 "Discovery of the Wilson Effect",

John Wiley & Sons John Wiley & Sons, Inc., commonly known as Wiley (), is an American Multinational corporation, multinational Publishing, publishing company that focuses on academic publishing and instructional materials. The company was founded in 1807 and pr ...

These observations were published in 1774 and 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 The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object of lesser size. In cases of equal or smaller impinging objects, only an umbra and penumba are generated. As ...

and umbra 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.

Alternative interpretations

While the surface-depression interpretation of the Wilson effect is widespread, Bray and Loughhead contended that "the true explanation of the Wilson effect lies in the higher transparency of the spot material compared to the photosphere". A similar interpretation was expressed by C.H. Tong.C.H. Tong (2005) "Imaging sunspots using helioseismic methods",

Philosophical Transactions of the Royal Society A ''Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences'' is a fortnightly peer-reviewed scientific journal published by the Royal Society. It publishes original research and review content in a w ...

363:2761–75

References

* C.A. Young (1882) ''The Sun'', page 126,

Kegan Paul Charles Kegan Paul (8 March 1828 – 19 July 1902), usually known as Kegan Paul, was an English author, publisher and former Anglican cleric. He began his adult life as a priest of the Church of England and held various ministry positions for m ...

. {{The Sun Solar phenomena

History

Alternative interpretations

See also

References