Augustus’ solar meridian functioning and the birth of the western leap year

In 12 BCE, Augustus undertook the responsibility for the calendar, which had gradually fallen out of alignment with the true dates of solstices and equinoxes. Augustus’ calendar reform, entailing the introduction of a leap day every four years, coincided with the erection of a grand meridian in the Campus Martius, known in Latin as Horologium Augusti. This device utilized the obelisk of Psamtik II (6° century BCE) as its gnomon that casted its shadow upon a travertine floor inscribed with bronze reference marks. Despite the discovery of the obelisk in 18th century and partial excavation of the floor in 1980, comprehending its geometric intricacies, regarding both dimensions and positioning, remained a challenge due to the complexities of conducting precise geomatic measurements in subterranean environments. Consequently, uncertainties persisted regarding its operational mechanics, particularly regarding whether the marks denoted days or ecliptic degrees. This study presents accurate measurements and statistically rigorous analyses that enable a precise repositioning of the meridian and a careful reconstruction of its geometry. The results suggest that the marks likely denoted specific days of the year, as recalled by Pliny the Elder. This provides support to the hypothesis that the monument functioned as an empirical validation of Augustus’ calendrical reform. In addition, the great accuracy achieved in positioning represents a fundamental aid in the desirable scenario to continue excavations of the meridian.