Roccabruna is a large quadrangular masonry 'cube' with a small Temple on the upper floor which has now disappeared.
On the lower floor there is a large circular hall – nine meters in diameter – covered by a closed dome.
The hall is accessed from the main door of the building, above which there is a rectangular opening of a window-tunnel (like in medieval castles), which inside the dome becomes a narrow slit.
On the days of the Summer Solstice, at sunset, the Sun's rays enter through that slit - which was a 'luminous conduit' - and create a Blade of Light inside the dome: it moves along an arc, turns red and then fades out.
The Sun sets in the center of the main door and illuminates the niche in front of it.
This "magic illumination" can be seen from 7.30pm to 8.45pm, only on the days of the Summer Solstice (from 19th to 23rd June) because the building is astronomically oriented at 300°, to capture the light only from that point.
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As we explain in our book «Villa Adriana. Architettura Celeste. I segreti dei Solstizi», the Blade of Light was a Secret, a sacred luminous signal that manifested the presence of divinity.
A candelabrum with the symbols of the Egyptian goddess Isis was found in Roccabruna: the building was probably dedicated to her cult, that overlapped the cult of the italic goddess Fors Fortuna, celebrated by the Romans in the days of the Summer Solstice (21 June).
These illuminations (hierophanies) studied by Archaeoastronomy give us a new key of interpretation to understand the function and symbolic meaning of Roccabruna and the Accademia, which stand on the highest artificial terrace of the Villa: the Accademia Esplanade. That was a real acropolis, a sacred area dedicated to mystery cults, with which Hadrian was affiliated.
Unfortunately I was never able to show the public this extraordinary magic illumination created by Emperor Hadrian, which stands alongside those that I discovered in the Pantheon (Arc and Square of Light) and in Castel Sant'Angelo in Rome (Rectangle of Light), and published in two other books, with Rirella Editrice.