Since we're living here on the Janiculum Hill, it's a 5-minute walk at midday for the ceremonial cannon shot just below Piazza Garibaldi.
You can read all about it via the links above, but what happens is this:
About 11:45 each morning, an olive-green van drives up with soldiers inside. Crowds gather around, both above on the piazza and down below as the soldiers unlock the door and drag this WWII vintage cannon outside.
There's some standing around and smoking of cigarettes, then the ceremony begins with soldiers flanking the cannon. The call is given to load the big gun and a charge is inserted. Some salutes are made and a countdown starts with 15 seconds to go. By this time the crowds are thick, especially above, where the view of Rome is extraordinary. At exactly 12 noon, BOOOOOOOOOOMMMM!!!! and then the church bells ring all across the Eternal City. Depending on the weather and crowds, the soldiers pose for photos for awhile before wheeling the cannon back inside, piling into their van and heading back to wherever they are based.
We spent the month of January at the Academy a few years ago, back when the gatekeepers went to lunch precisely at noon. We lived across the street and were told that the gates could not be opened for anyone to get in or out while the gatekeeper took his 30 minutes for lunch. This meant if you were inside and wanted to be across the street in your apartment, you'd better get through the gate BEFORE you heard the cannon shot! Both yours truly and Heather lost track of the time now and then and were forced to wait until the gatekeeper returned. These days those living here have special electronic keys to get in and out and there's a backup gatekeeper, just-in-case.
Some detail photos of the monument in the center of Piazza Garibaldi