The Obelisk of Montecitorio was used as the sundial for the Horologium Augusti in Rome. It is 21.79 meters high and made of Aswan granite. It was originally erected in Heliopolis in the early 6th century BC by Psamtik II. It was one of two carried off by Augustus in 10 BC with identical inscriptions - the other being the obelisk at the Circus Maximus. 
It was erected by Augustus in 10 BC on the twentieth anniversary of his conquest of Egypt. It was made the gnomon of a colossal sundial designed by the mathematician Facundus Novius. Its purpose was to demonstrate the accuracy of the new calendar introduced by Julius Caesar (which inserted an extra day every four years). It was part of the Augustan complex that included the Ara Pacis and the Mausoleum Augusti. The obelisk was presumably fitted with a gilded sphere surmounted by a short spire at its apex and probably mounted on a stepped base. There was network of bronze markers on the pavement to show the length of the shadow throughout the year and inscriptions indicating the seasons and signs of the zodiac and various meteorological phenomena.
Pliny observed that by his day the readings had been inaccurate for thirty years, apparently due to settling of the obelisk in the soft soil of the Campus Martius. The meridian line was reset, presumably during the reign of Domitian. 
The obelisk was still standing in the eighth century and was later discovered in the early 16th century. It was excavated in 1748 and reerected in 1792 in Piazza di Montecitorio. It was broken when it was discovered and was repaired using parts of the Column of Antoninus Pius. 

See also Roman Monuments and Roman Obelisks