Obelisk of Montecitorio

The Obelisk of Montecitorio (Italian: Obelisco di Montecitorio), also known as Solare, is an ancient Egyptian, red granite obelisk of Psammetichus II (595-589 BC) from Heliopolis. Brought to Rome with the Flaminio Obelisk in 10 BC by the Roman Emperor Augustus to be used as the gnomon of the Solarium Augusti, it is now in the Piazza Montecitorio. It is 21.79 metres (71 ft) high, and 33.97 metres (111 ft) including the base and the globe.

Augustus erected it as the gnomon of the Solarium Augusti, his giant sundial (or horologium) in the Campus Martius. The meridian, worked out by the mathematician Facondius Novus, was placed in the center of a surface measuring 160 by 75 metres (525 by 246 ft), constructed from slabs of travertine, on which a quadrant was marked out with bronze letters, with indications of the hours, months, seasons and signs of the zodiac. Besides its function as a solar clock, the obelisk was oriented in such manner so as to cast its shadow on the nearby Ara Pacis on 23 September, Augustus's birthday, which coincided with the autumnal equinox.

A detailed description that gives us the typology, appearance and formal operating procedure of this imposing solar meridian is supplied from Pliny the Elder (Naturalis Historia 36, 71–72).

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