Short answers, pulled from the story.
Emperor Augustus erected the Solarium Augusti in 10 BCE within the Campus Martius of ancient Rome. The monument served as a gnomon that cast shadows across a marble pavement featuring an intricate network of lines made from gilded bronze.
The thirty-meter Egyptian red granite obelisk from Heliopolis functions as a gnomon to track the changing noontime position of the Sun throughout the year. Modern research suggests it operated primarily as a meridian line rather than a full sundial, allowing observers to determine the date by watching where the shadow fell upon specific markers.
Augustus aligned the Solarium Augusti with the nearby Altar of Peace so that on September 23, which was the birthday of Augustus himself, the shadow of the gnomon fell across the center of the altar. This specific alignment linked the emperor's birth to the concept of imperial peace and connected military authority with the promise of stability.
Pliny the Elder recorded details about the monument in his Natural History text stating that the structure ceased to reflect the solar year accurately by approximately 40 CE. He proposed multiple explanations for this shift including theories that the sun might not be fixed or that the earth itself could have moved differently than expected.
The recovered piece is now known as the Obelisk of Montecitorio and stands in Piazza di Montecitorio where Pope Pius VI re-erected the monument in 1789 during a triumphant rededication ceremony. The ancient inscription remains visible on the restored structure today while a reclining figure once supported the base before it was moved to the Vatican Museums.