Il regolo lunare di Palazzo Spada, Roma. Indagine su un errore

A first part of historic-artistic character of the present study introduces the catoptric clock of the Palazzo Spada and the tools related to it. The so-called lunar regulus is then examined, a tool that would permit to know the solar hours even at night thanks to a simple conversion, but which, according to some scholars, do not provide correct results. The authors discuss the motion of the moon with respect to the sun and go through the direct ancient and modern sources with the goal of making understandable the operation of the instrument and discover the origin of the errors. In the Appendix a teaching tool derived from this research is presented.