Buchner's reconstruction of the Horologium Augusti continues to be influential, despite fatal flaws demonstrated by Rodríguez-Almeida and Schütz; so we begin by reviewing the state of the evidence. There is no credible indication that Augustus or his successors built an extensive sundial, so we must conclude with Schütz that what Pliny described and what the excavations of the German Archaeological Institute (DAI) uncovered was a solar meridian line. We then proceed to discuss its significance in the context of the Augustan building programme. Since the DAI in fact discovered a Flavian reconstruction, the final question to be addressed is what purpose it served Domitian to rebuild a largely symbolic Augustan instrument that had grown inaccurate over time.