It is also a sun dial, its shadows mark noon over the signs of the zodiac in the white marble disks in the paving of the square. The obelisk rests upon four couchant lions, each with two bodies whose tails intertwine.

The Obelisk from Egypt was brought to Rome by Emperor Caligula in 37 AD. It originally stood in his circus on a spot to the south of the basilica, close to the present Sacristy.