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. The
solarium was dedicated to the
Sun in 10 BCE, 35 years after
Julius Caesar's calendar reform. It was the first solar dedication in Rome. 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. The obelisk itself was set up to memorialize Augustus' subordination of Egypt to the control of the Roman empire. The two monuments must have been planned together, in relation to the pre-existing
Mausoleum of Augustus, to demonstrate that Augustus was "born to bring peace", that peace was his
destiny. According to the
Cambridge Ancient History, "the collective message dramatically linked peace with military authority and imperial expansion." 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. The obelisk was illustrated, supported by a reclining figure, on the base of the
Column of Antoninus Pius. The obelisk gnomon was still standing in the 8th century CE, but was thrown down and broken, then covered in sediment; it was rediscovered in 1512, but not excavated. In a triumphant rededication, the '
Montecitorio obelisk' was re-erected in
Piazza di Montecitorio by
Pius VI in 1789. In 1979, a section of the ancient paving of the square was found at a depth of 8 metre pit inside a series of vaults in the Campo Marzio, bearing a calibrated line and inscriptions in Greek referring to zodiac symbols. The ancient consecration inscription has been maintained and reads: == Archaeology ==