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Largest sundial (Solarium Augusti)

This post is also available in: Polish (polski)

Horologium solarium Augusti
Augusti's solarium horology

Horologium Solarium Augusti was the largest sundial and calendar of all time. The monument was located in Rome on Field of Mars. Currently, this obelisk is located in Piazza Montecitorio in Rome in front of the Chamber of Deputies.

For its construction, an obelisk brought from Heliopolis in Egypt by Octavian Augustus was used, which was to be a symbol of Egypt’s subordination to Rome. This obelisk was 30 meters high and was made of red granite. He played the role of a gnomon or a vertical pointer, whose shadow cast on a shield placed in the horizon plane indicated time. The shield was a marble floor, which had bronze lines on which to base the time of day.

The whole building was crowned with a sphere, which symbolized the whole world. This clock began to be erected with the construction of Ara Pacis in 13 BCE. The shadow of the obelisk on the birthday of Emperor Agust – September 23 – pointed to the heart of the altar. This clock together with Ara Pacis heralded a new era of peace, the time of Augustus.

The building was dedicated to the deity Sol in 10 BCE and it was the first “gesture” of this type towards this god. The inauguration of the building probably took place in the same year as the dedication Ara Pacis – 9 BCE. It is worth noting that the same year there was a correction of the Julian calendar – it turned out that the leap year occurred every third year and not every fourth one as assumed by the calendar.

Later, Pliny the Elder mentioned that the clock did not indicate the time of day and was inaccurate, which he tried to explain by the fact that the earth could have fallen as a result of earthquakes and floods.

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