umbra recta

Etymology
First attested in Modern English in 1688, although the phrase was also used in by Geoffrey Chaucer (c. 1343–1400) in his Treatise on the Astrolabe; :  +  (feminine nominative singular of, “straight”) = “straight shadow”; compare.

Noun

 * 1) On a shadow in the shape of a right triangle, the length of the shadow's edge adjacent to a measured angle.