Follow the gallery around for about a thousand paces until you come to the hypethral. With it dark out you might miss it, so keep an eye open for the plants.
-- Gene Wolfe, Shadow and Claw
The choice of top light for the main galleries is said to have been dictated by the belief that Greek temples were hypethral, that is, open to the sky; from which it was inferred that Greek taste demanded to see works of art under light from above.
-- Benjamin Ives Gilman, Museum Ideals of Purpose and Method
Hypethral stems from the Greek roots hyp- which means "under" and aîthros meaning "clear sky."