| Term: | Wilson effect |
| Definition: |
A phenomenon in which the shape of sunspots flattens as they approach the Sun's limb due to the solar rotation. More specifically, when a sunspot approaches the solar limbs the width of the penumbra, relative to the umbra, on the side facing the center of the Sun seems to become narrower than on the side facing the limb. This phenomenon arises from a projection effect, and is due to a geometrical depression (the Wilson depression) in the layers of constant optical depth in sunspots. |