Wilson effect
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In 1769 a Scottish astronomer named Alexander Wilson noticed that the shape of sunspots noticeably flattened as they approached the Sun's limb as the Sun rotated. These observations showed that sunspots were features on the solar surface, as opposed to minor planets or objects situated above the surface. Moreover, he concluded that sunspots were in fact slight depressions in the surface of the photosphere. Sunspots occur when the heat from the core is stopped by the sun's own magnetic field. Sun spots are much cooler than the rest of the photosphere and form at a temperature of about 7000°F (about 4000°C). Sunspots have high and low periods that rotate every 11 years.Template:Astro-stub