Solarium Augusti

The Solarium Augusti, also called the Horologium Augusti, was an ancient monumental sundial in the Campus Martius area of Rome established under Emperor Augustus. It combined a long pavement cursus and an upright obelisk used to cast a shadow for calendrical and ceremonial purposes.

The monument incorporated an Egyptian obelisk as its gnomon and a marked pavement or line intended to indicate the sun’s position on key dates such as solstices and possibly Augustus’s birthday. Fragments and later mappings of the site give estimates of a considerable urban scale rather than a small garden device.

Built under the authority of Augustus in the late 1st century BCE, the horologium formed part of a suite of public works that linked imperial ideology with Rome’s urban fabric. Over subsequent centuries much of the pavement and associated structures were lost or buried; the obelisk was moved in the medieval or early modern period and re-erected in a different Piazza.

The original installation stood on the Campus Martius within the modern city of Rome, in the area now occupied by civic streets and squares near the Piazza di Monte Citorio.

• Obelisk gnomon: Known as the Horologium Augusti, it used an ancient Egyptian obelisk as the gnomon to mark solar events and monumental time points in Rome.