The sundial of Augustus and its survey: unresolved issues and possible solutions

A lively debate has developed regarding the characteristics of the so-called Horologium Augusti, at first known only through a notice in Pliny and subsequently discovered (at least partly) during the course of excavations begun in 1997 (Leonhardt, in: The Horologium of Augustus: debate and context, 2014). The gnomon of the “Horologium” was composed by the obelisk that presently is nearby in “Piazza Montecitorio” in Rome (Fig. 1). A large part of the debate has centred on the very function of the Horologium, in particular whether it was a true functioning solar clock or simply a sundial. The scope of the present essay concentrates rather on the metrical accuracy that such a sundial could have had; in particular, we will hazard a hypothesis as to the accuracy with which the direction of the sundial was laid out and the possibility of measuring the azimuth in its present placement. Such a detailed geodetic-topographic survey of the portion thus far excavated, could provide useful information for the eventual pursuit of excavations yielding, at the same time, further avenues of research; as an example it would also allow for the deduction of two pieces of information still not entirely established: the exact height of the gnomon, and the exact position of the original placement of the axis of the obelisk.