It should be plain to see that the obelisk inside the ellipse provides a sundial, and marks the equinox sunrises (March and September) in the east. This is appropriate since obelisks represented rays of the sun and were dedicated to the Egyptian Sun god Ra.

The Washington Monument provides a sundial in DC, but it is located west of the Capitol, so it is the sunset that is marked there.

The obelisk at St. Peter's was brought to Rome by Caligula and set up in the Vatican Circus, which was finished by Nero. Pope Sixtus V moved it to it's current location in 1586.

The Roman Circus was a place where chariot races took place around a central obelisk, which symbolized the sun as the axis of the solar system. The race course was likened to the path of the planets around the sun, with smaller obelisks marking the solstice limits of the ecliptic path. Looking at the top image on this page you will see that the small obelisks were replaced by fountains.

Many Christians were martyred in the Vatican Circus, among them St. Peter, who is supposed to have been killed near the obelisk. Constsantine founded the most holy of Christian Churches here, because Peter was said (by Constantine) to have died and been buried here. The image below compares the location of the circus and the basilica.

The fact is that other obelisks are located in New York, Paris and London, and they were placed on pedastals like below.

While the Egyptians had perfected the method of balancing the obelisks on their end on a level platform, the Romans placed iron crabs at the corners when they moved the obelisks. When the move was made to St. Peter's, lions were used.

The pedastal for the Vatican obelisk is composed of three pieces of stone.

The next image shows the scaffold used to erect the obelisk onto the pedastal at the Vatican.

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