Adjective
umbral (not comparable)
Of or pertaining to an umbra.
Contact points relative to the Earth's umbral and penumbral shadows, here with the Moon near is descending node The timing of total lunar eclipses are determined by its contacts: citation :P1 (First contact): Beginning of the penumbral eclipse. Source: Internet
Lunar eclipse appearance A lunar eclipse occurs in two regions, an outer penumbral shadow where the sunlight is dimmed, and an inner umbral shadow, where much dimmer sunlight only exists by refraction through the Earth's atmosphere, leaving a red color. Source: Internet
The analogous formulas for the backward and central difference operators are : The calculus of finite differences is related to the umbral calculus of combinatorics. Source: Internet