Which giant sundial utilized an obelisk as its gnomon?

The giant sundial that utilized an obelisk as its gnomon is the Horologium Augusti. This monumental structure was built during the reign of Emperor Augustus in the Campus Martius in Rome. The sundial's obelisk, originally from Heliopolis in Egypt, was positioned vertically and cast a shadow onto a carefully arranged circular area that marked the hours of the day.

The construction of the Horologium Augusti showcased Augustus's desire to connect his reign with the grandeur of ancient Egypt and demonstrated the engineering and architectural capabilities of the Roman Empire. The use of an obelisk as a gnomon was both a functional choice, allowing for accurate timekeeping based on the position of the sun, and a symbolic choice, reinforcing the concept of Roman dominion over Egypt and the connection to the divine.

In contrast, while other options like the Horologium Solis, Horologium Aurelianum, and Horologium Trajanum may refer to sundials or related solar structures in Rome, they did not prominently feature an obelisk as their gnomon or are less well-documented in their connection to the famous sundial attributed to Augustus. Thus, the prominent historical significance and design of the