theory of sundials

“Midsummer Twilight” by Willard Metcalf

From astronomy we find the east, west, south, and north, as well as the theory of the heavens, the equinox, solstice, and courses of the stars. If one has no knowledge of these matters, he will not be able to have any comprehension of the theory of sundials.
~ Vitruvius Pollio
(Vitruvius, the Ten Books on Architecture)