goes to Horologium 5386y
Augustus brought two obelisks from Heliopolis to Rome, one of which was set up in the Circus Maximus and the other in the campus Martius, between the ara Pacis Augustae and the columna Antonini Pii. The latter is the one now standing in the piazza di monte Citorio. The inscriptions on the pedestals of the two obelisks are identical, and show that they were set up in the year 10 B.C.
The obelisk of the campus Martius is of red granite, 21.30 meters in height and covered with hieroglyphics. The travertine base has been much restored, and the column itself repaired with fragments of stone from the columua Antonini.
Augustus employed this obelisk as the gnomon, or needle, of a great sun-dial, or solarium, formed by laying an extensive pavement of white marble on the north side of the obelisk, on which pavement the lines were indicated by strips of gilt metal inlaid in the marble. Portions of this pavement, of the gilt lines, and of figures of animals representing the signs of the Zodiac, were found at various times in the fifteenth and sixteenth centuries, but they were again covered and are still buried beneath the modern buildings. The obelisk itself was thrown down at some unknown date, and although it was discovered under the ground in 1463, it was not excavated until 1748, nor set up until 1792. The marble pavement must have extended about 150 meters in an east and west direction, and something more than half that distance north and south.
Vincenzo Fasolo, "The Campo Marzio of G. B. Piranesi".