Solarium Augusti

It was erected by the emperor Augustus, with the 30-meter Egyptian red granite Obelisk of Montecitorio, that he had brought from Heliopolis in ancient Egypt.

The obelisk was employed as a gnomon that cast its shadow on a marble pavement inlaid with a gilded bronze network of lines, by which it was possible to read the time of day according to the season of the year.

[4] The Solarium Augusti was integrated with the Ara Pacis in the Campus Martius, aligning with Via Flaminia, in such a way that the shadow of the gnomon fell across the center of the marble altar on 23 September, the birthday of Augustus himself.

"[6] In his Natural History, Pliny remarks that in the monument had stopped accurately reflecting the solar year by about 40 CE and offers several explanations for the shift, including that the sun, the earth, or both might not be as fixed in their position and movement as was usually believed at the time.

Recent studies have challenged Buchner's reconstruction of the Solarium as a full sundial, maintaining that the archaeological and textual evidence indicates a simple meridian line, marking the changing noontime position of the Sun in the course of the year.