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NOAA - August 2023


The air in the French Alps was unusually steady on Aug. 15th when Thierry Legault pointed his telescope at the sun and captured a phenomenon rarely seen by amateur astronomers. It's called the "Wilson effect:"


"The sunspot's umbra is lower than the average solar surface, with surrounding penumbral filaments dipping down into it," says Legault. "The filaments are visible on the far edge of the 'bowl,' but not on the near edge, highlighting the depression."

Scottish astronomer Alexander Wilson discovered the effect in 1769 during Solar Cycle 2. In daily observations, he noticed that sunspots approaching the sun's limb were foreshortened and often appeared to be sunken or depressed in the middle. (The same observations proved that sunspots were features on the solar surface, not, say, dark satellites orbiting just above the sun.)

The Wilson effect has been debated for more than 250 years. Is it real? Some researchers in the 1950s argued that it was a purely psychological effect. Others retorted that observers couldn't be crazy; there was too much photographic evidence. Although the cause of the Wilson effect is still debated, many modern researchers take it at face value: Sunspots are shallow depressions. One idea holds that high pressure systems trapped in the magnetic canopies of sunspots press down on the underlying umbra, creating a dark bowl.

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