From  13 to  9  BC  Augustus  initiated  projects  in  the  Northern  Campus  Martius  to  erect  the  Ara  Pacis, Montecitorio Obelisk, and the gnomonical device generally known as the Horologium Augusti.2 The projects had two motivations closely related to the two ways we can couple the monuments: obelisk—gnomonical device and obelisk—Ara Pacis. The obelisk—gnomonical device pair is functionally bound, serving the purposes of accurate time-keeping. The obelisk supported a sphere on a rod attached to the top of its pyramidion, and the shadow of the sphere hit a crosshatch inscribed on the meridian line at local noon each day of the year. The obelisk—Ara Pacis pair appears to have served the purpose of expressing the symbolism of imperial ideology, since  the  two  monuments  are  visually  bound but  have  no  functional  connection.  They  have  a  similar orientation, and the obelisk stands  athwart an extension westward of  the altar’s axis  of symmetry.  In recent years, the attention of scholars has been concentrated more on the first than on the second pair.
The article takes as its point of departure recent work (Frischer forthcoming) critiquing the theory of Edmund Buchner about the relationship of the gnomonical instrument known as the Horologium Augusti and the Ara Pacis Augustae. As a result of this critique, the Montecitorio Obelisk could be situated with greater precision on the map of the city. A computer simulation showed that Buchner erred in positing that the shadow of the Montecitorio Obelisk went into the center of the Ara Pacis on Augustus’ birthday. In this article, computer simulations are used to develop a post-Buchnerian interpretation of the relationship of the obelisk and altar. Over 230 hitherto unrecognized solar and shadow alignments are reported. The first part of the article defines four zones around the monuments where the solar and shadows observations were made. In the second part of the article, specialists interpret the significance of the annual solar and shadow spectacle from various points of view. The conclusion synthesizes the results, arguing that the monuments were intentionally aligned and situated in order to propagate the same message as the one inscribed on two sides of the Montecitorio Obelisk [CIL 6.702 = ILS 91]: that Augustus was a devoted worshipper of the sun god (Sol), who brings Rome victory in war, peace, and prosperity through his earthly representative, the emperor.