Obelisk of Psametik II from Heliopolis
reerected by Augustus in northern Campus Martius

Purpose of the obelisk? This obelisk, erected by Augustus in the northern Campus Martius in 10-9 BCE, had been quarried nearly 6 centuries before at Aswan, Egypt. The obelisk had been erected by Psammetichus II (Psametik II), reigned 594-589 BCE, as one of a pair of obelisks, dedicated to the sun god at the sanctuary to Re in Heliopolis. It was removed from Heliopolis by Augustus, shipped across the Mediteranean, and reerected in the northern Campus Martius 10-9 BCE as a single obelisk, also dedicated to the sun. This was the first of several obelisks removed from Egypt and reerected in Rome. It has been seen both as a symbol of the Roman conquest of Egypt and as a symbol of the inclusion of Egypt as part of the Roman empire.

As reerected by Augustus, the obelisk was mounted on a new rose granite base, quarried from the same source as the original obelisk. The base was then inscribed on 2 sides (see photos and text next page) and a metal globe with spire attached to the obelisk's pinnacle. A travertine pavement with a single bronze guideline, with cross markers, known as the meridian, was installed, running approximately north of the obelisk. These cross lines marked the 360 degrees of the solar year. The obelisk then served as the pointer or gnomom of a sundial that tracked the length of the obelisk's shadow each day at noon. This made it possibe to confirm the accuracy Augustus' correction to the 365 1/4 day calendar. The obelisk also served as a fulcrum, helping to tie together the three major Augustan monuments in the area: the obelisk, Ara Pacis, and, most prominently, the Mausolum of Augustus.

For years, many scholars accepted the proposal that the obelisk was not only the pointer for a sundial tracking the changing length of the obelisk’s shadow each day along a meridian, but was in addition the gnomon for a complex horologium-solarium, with bronze rods projecting across a vast travertine pavement. For years it was so represented in most diagrams, paintings, and models. Partly because no-one has been able to find evidence to support this idea, it has gradually been discounted by scholars.