The apparent displacement of a sunspot's umbra towards the Sun's centre as the spot approaches the limb, named after the Scottish astronomer Alexander Wilson (1714–86), who first observed it in 1769. The effect gave rise to the belief that sunspots were saucer-shaped depressions. However, good examples of the effect are few. Sometimes, displacement of the umbra away from the Sun's centre is seen (the reverse Wilson effect). The effect could be explained by the fact that spots rarely have a circular outline.
Subjects: Astronomy and Astrophysics.