The notions of the penumbra and umbra are important in considering eclipses (q.v.).
Gradually I emerged from the penumbra of that experience with a mind made clearer by trial and with a truer knowledge of life.
He pointed out that they were limited to a certain defined zone on the sun's surface; he noted the faculae with which they are associated, the penumbra by which they are bordered, their slight proper motions and their rapid changes of form.
When the eclipse is total, there is a real geometrical shadow - very small compared with the penumbra (for the apparent diameters of the sun and moon are nearly equal, but their distances are as 370: I); when the eclipse is annular, the shadow is all penumbra.
The breadth of the penumbra when the source and screen are nearly equidistant from the opaque body is equal to the diameter of the luminous source.
In the neighbourhood surrounding the penumbra the granules appear to be packed more closely, forming brilliant patches called faculae.