An Etymological Dictionary of Astronomy and Astrophysics

1. Optics: A dark area, especially the blackest part of a shadow from which all light is cut off. → penumbra.
   

2. Sun: The central darkest part of a → sunspot, which is also the coolest part (~ 4,200 K).

Etymology (EN): From L. umbra “shade, shadow.”

Etymology (PE): Sâyé “shadow,” from Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;”
M.H.G. schinen, O.H.G. skinan, Ger. Schein “glow, shine;” PIE base *skai- “bright.”