Abstract

Three small fragments (see figure) of a light limestone vertical deviating sundial, now in the City Antiquarium of Guidonia Montecelio, were found on different occasions in the countryside of Guidonia, a small town near Tivoli. The fractures of two of these pieces match, so they can be put together, while the third piece is from a different part of the sundial. The reassembled fragment is 115 mm high, 200 mm wide and 7 mm deep. The dimensions of the detached fragment are 146 mm by 16 mm by 27 mm. The noon hour line is completely missing, so it is difficult to make the usual calculations to determine the intended latitude. Consequently, the author has attempted an inverse procedure from the one normally followed when studying a sundial: the object was reconstructed starting from the reasonable hypothesis that it was built for the latitude of 42°, the latitude in which it was found. The angle between the equinox line and the upper edge of the fragment, assumed to be horizontal, was measured and found to be 40°, from which a deviation 49° west can be deduced. On the basis of these figures, the author made a drawing of a vertical deviating sundial which would seem to correspond quite well if the perpendicular height of the gnomon was about 130 mm. In the example, the winter hyperbola appears higher than that calculated astronomically, a situation that is not rare in ancient sundials of imprecise workmanship. Through the reconstruction it was possible to identify, with good approximation, the position originally occupied by the third fragment, which contains a small part of the summer solstice curve. The shape of the cut stone slab must have been something like a triangle, with two straight sides ¬- the upper horizontal and the noon vertical - and the third an arc approximately parallel to the summer solstice curve. It is clear, in fact, from the third fragment that this arc is not the result of a fracture but was carefully worked by the builder, who engraved an X in the concavity, rotated by 90° and delimited by two small vertical bars.