Anew study apparently settles a long-standing mystery of the Ara Pacis ("Altar of Peace"), which dates back to the year 9 B.C. and was built to honor an era of peace ushered in by Rome's first emperor Augustus, according to LiveScience. That era was known as the Pax Romana. The Ara Pacis building on the Tiber River doubled as a huge sundial —or gnomon —with the help if a nearby Egyptian obelisk taken from Heliopolis. Historians have long thought that the sun would line up with the obelisk and Ara Pacis to cast a shadow in the building's plaza on Sept. 23, the birthday of Augustus. But using computer simulations and NASA's star-plotting Horizons System, the scientists discovered the alignment actually occurs on Oct. 9, which is the annual festival of the Temple of Palatine Apollo.