noun
Astronomical term for the partial shadow during an eclipse, where the light source is only partially blocked.
月食のとき、月が半陰影に入ると少し暗く見える。
During a lunar eclipse, when the moon enters the penumbra, it looks slightly darker.
本影 is the umbra, the fully dark inner shadow, while 半陰影 is the penumbra, the partial outer shadow.
Compound of 半 (half) and 陰影 (shadow), directly corresponding to the astronomical concept of penumbra.