Abstract

A light-colored marble vertical deviating sundial, in a good state of conservation (see figure), was found on the site of some Roman buildings in the course of recent roadworks at Passo Corese in the Municipality of Montelibretti, some 50 km North of Rome. [...]. The sundial is inscribed with the winter solstice curve, completely preserved; the summer solstice curve, about half of which remains; and the equinox line, almost all of which remains. There are six hour lines. The usual methods of calculation give an intended latitude of about 39°, little differing from that of the place where it was found, namely 42°. To judge from the slope of the equinox line, the face of the sundial probably deviated from the prime vertical by 36° west. The length of the gnomon, according to the calculations, was 58 mm. The test of the distance along the equinox line between the noon line and the first hour after noon gave the following results: calculated 35 mm, measured 44 mm. Taking everything into account, the conclusion is that this sundial was built with a relatively good degree of accuracy.