Only in the week of the Summer Solstice (from 19 to 24 June) in the Hall od the Burial Urns of the Mausoleum of Hadrian, today’s Castel Sant'Angelo, there are special illuminations which had a precise symbolic meaning.
The Hall of the Burial Urns has three niches on the east, west and north sides, for the sarcofagi of emperor Hadrian, his wife Sabina and probably Aelius Caesar, the designated heir who died before them.
The Sun enters through the two window-tunnels that open into the vault of the Hall of the Burial Urns, creating rectangles of light in each of the three niches.
The west niche is illuminated in the morning, at 8.30am before opening hours.
The central north niche was illuminated twice, at 10am, and 4pm, but the rectangle today hits the masonry walkway that crosses it and therefore can no longer be seen.
Finally, the east niche is perfectly centered by the rectangle of light just before 5pm. The rectangle is divided in two by the shadow of the statue of the Archangel Michael which is located outside, right in front of the window.
The large window-tunnels from which the light enters open high up into the Cortile del Pozzo and the Cortile dell'Angelo, and were designed to capture the sun's rays only on those days.
The openings also in the floor of the Courtyards were designed for this purpose, transforming them into sort of "death traps" protected by grates and railings to prevent visitors from falling into them.
The illuminations that appear only during the days of the Summer Solstice were a sacred luminous signal of the favorable presence of the divinity.
The symbolic meaning of these magic illuminations is revealed in the book by Marina De Franceschini and Giuseppe Veneziano «Castel Sant'Angelo. Hadrian's Mausoleum. Architecture and Light» also in english edition, which retraces the thousand-year history of the Mausoleum and proposes a new reconstruction of its ancient appearance.