An Etymological Dictionary of Astronomy and Astrophysics

A dark patch formed by a body which obstructs rays of light.

Etymology (EN): M.E. sch(e)adew(e), schadow, shadw(e),
O.E. sceadwe, sceaduwe, sceadu “shade, shadow, darkness;” cf. O.S. skado, M.Du. scade, Du. schaduw, O.H.G. scato, Ger. Schatten, Goth. skadus; from PIE base *skot- “dark, shade.”

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.”