In 10 BC Augustus had the obelisk of Psammeticus II (6th century BC) removed from Heliopolis. This was a 22-meter-high monolith in red granite with hieroglyphics on the four sides. The obelisk was placed in the northern Campus Martius on a base inscribed with a dedication to the Sun by Augustus: “Imp. Caesar divi fil. / Augustus / pontifex maximus / imp. XII cos. XI Trib. pot. XIV / Aegypto in potestatem / populi romani redacta / Soli donum dedit.” In this way the obelisk became the gnomon of the Horologium Augusti, the greatest sundial ever made. The obelisk-gnomon was about 30 meters high from its base to its top, crowned by a globe. We do not know when or why it collapsed and broke into five pieces.

We just know that its fragments were found by chance in 1502 in the basement of a barber’s shop in Largo dell’impresa, a no longer existing little street, and that three centuries elapsed before it was re-erected by Pope Pius IV in Piazza Montecitorio in 1792.