It’s not a square. Piazza is translated “square” but is properly a place for gathering or a marketplace. Piazza San Pietro is an ellipse. A circle has one center and any point on the circle is equidistant from the center. An ellipse has two foci and major and minor axes. If an ellipse collapses to a circle, the two foci would come together in one point. An ellipse has the property that the sum of the distance from one focus to any point on the ellipse and from there to the other focus is equal to the length of the major axis. Anyway, you probably know what an ellipse is. The Bernini Columns are the arms of Saint Peter’s Square that welcome the world to the church. The following picture was taken in October 2004 when Bette and I visited Rome on the way back from an SPE meeting. We haven’t yet climbed to the top of St. Peter’s dome this time in Rome. Perhaps we will climb it this week.
The columns are actually four rows of columns that form four ellipses of ever larger dimensions. All four ellipses have the same foci. This has the interesting property that if you stand on one of the focus points, you see only one row of columns on your side of Saint Peter’s Square. The other three rows of columns are hidden, exactly behind the one you see. There are stones in the pavement to mark the two focus points.
There are other interesting stones in the pavement. They mark the solstices and each month between the winter and summer solstice. From the photo above, you can see that there is a large obelisk in the center of Saint Peter’s Square. On a sunny day, that obelisk casts a shadow. We were not able to go to St. Peter’s square on the summer solstice, but made it there on June 23rd. My astronomy software said that the sun would transit (reach its highest point in the sky) at 13:12, so I went to Saint Peter’s Square to see the transit. I took the following photo of the shadow of the obelisk on the summer solstice stone. I was probably thirty seconds late, because I was explaining the event to some other tourists.
The obelisk is actually 2100 years old. It was first set up in Heliopolis by the Roman Curate there. It was brought to Rome by Emperor Caligula in 38 AD and set up in a circus (horse race stadium) near the present site of Saint Peters. It was moved to Saint Peter’s Square when the square was built in 1586 under the direction of Pope Sixtus V.