The notions of the penumbra and umbra are important in considering eclipses (q.v.).
The umbra in the former case becomes the fully illuminated portion, and vice versa.
Each spot shows with more or less completeness a ring-shaped penumbra enclosing a darker umbra; the umbra, which looks black beside' the photosphere, is actually about as brilliant as limelight.
In a partial lunar eclipse, it partly enters the umbra and only part of its surface is darkened.
When the slit of the spectroscope is set across a spot, it shows, as might be expected, a general reduction of brightness as we pass from the photosphere to the penumbra; and a still greater one as we pass to the umbra.