The heart of the complex was an enormous natural Grotto, the "spelonca" that originated the name Sperlonga, which was transformed into a spectacular and richly decorated nymphaeum.
In 1957, engineer Erno Bellante discovered the famous sculptures of Ulysses blinding Polyphemus, of Scylla, and other sculptures reduced to fragments now on display in the beautiful Museum of the archaeological site of the Villa of Tiberius in Sperlonga.
Inside the cave, Tiberius built a large circular basin surrounded by a platform for walking, connected to another rectangular basin that extended outside.
The Grotto is divided into two smaller grottoes, A on the left and B on the right,
which were originally much larger and irregular, with secondary
ravines. Their shape was regularized by building walls covered in pumice
stone that blend in with the natural rocks. And there he placed a
spectacular summer triclinium, overlooking the magnificent panorama with the promontory of Circeo, where according to legend the sorceress Circe lived.
At the end of grotto A, a door leads to a cubicle with three rectangular niches for beds
(in Roman houses in the Vesuvian area there are cubicles near the
triclinia, which were used to rest after lunch or dinner). The walls
still have fragments of the wall decoration with mosaic and shells.
Grotto B has a podium with steps that go up to the inner part, on a higher level. At the end, a large rectangular niche was dug, where in all likelihood the states of Ulysses blinding Polyphemus were placed, together with other sculptures of Homeric subjects found in 1957.
Thanks to Google Earth, we saw that the two grottoes had the same orientation (azimuth) of the sunset of the two Solstices. Grotto A is oriented towards the sunset of the Winter Solstice, December 21; grotto B is oriented towards the sunset of the Summer Solstice, June 21.
Since prehistoric times, the Solstices are the key dates most frequently chosen for astronomically oriented structures or buildings, so that illuminations took place there only in those days, and were a sacred signal of the presence of divinity.
On-site observations have confirmed our hypotheses of an astronomical orientation.
On June 19, 2019, standing in the center of the niche in grotto B we saw the Sun set in the center of the Cave. The same thing happened during the two public events organized in June 2022 and 2023 thanks to the Director of the Museum, Dr. Cristiana Ruggini.
Other observations on December 23 and 24, 2019 confirmed the orientation of grotto A, thanks to the cooperation of the very kind staff of the site, who took the photographs and to whom we are extremely grateful.
(Unfortunately in the months of November and December the site closes at 4:00 pm so it is not possible to observe the sunset from the Cave at 4:30. The same goes in the summer months, since the site closes at 7:00 pm and the illuminations are at 8:30 pm).
The statues of Ulysses recalled the divine ancestry of Tiberius and the Gens Claudia, who descended from Telegonus, the son of Ulysees and the sorceress Circe.
Only on the days of the two solstices did the light of the sun enter the grottos, illuminating the triclinium and guests attending the banquets. The illuminations were a sign of the divine presence, confirmed the divine origin of the emperor, and therefore legitimized his power as an intermediary between men and gods.
The Roman emperor was at the same time a political and religious leader, being Pontifex Maximus, the most ancient and important Roman priestly office. In that role Tiberius attended magnificent banquets in the Grotto, which had a religious and ritual symbolic meaning, underlined by the solstitial hierophanies.