1. The shortening of a sunspot lying close to the edge of the sun's visible disk.
2. An effect in which the penumbra of a sunspot appears narrower in the direction toward the sun's center than in the direction toward the sun's limb.

In 1769, a Scottish astronomer named Alexander Wilson noticed that the shape of sunspots noticeably flattened as they approached the Sun's limb due to the solar rotation.