"To the north there is another grand obelisk which is in fact the pointer of a giant sundial. This too was designed by Augustus and, on his birthday, the shadow of the obelisk points directly at the Ara Pacis, Augustus' altar of peace, one of the finest examples of Roman sculpture ever created. The altar dates from 9BC. It is enclosed by walls of white marble on which are carved bas-reliefs of garlands and flowers. Above this are reliefs of the imperial family, the priests, senators and Roman people going in procession to give thanks for the blessings of the Roman peace."
From the very amusing article "Roman holiday: A trip back in time".