Thanks to the cooperation with the Croatian scholar Zlatko Andrijašević – who had seen our discoveries on the Internet about the Arc and the Square of Light in the Pantheon at Rome – in 2021 we discovered the astronomical orientation of the Vestibule in Diocletian's Palace in Spalato (Split, Croatia).

During the days of the Summer Solstice, the oculus of the vestibule dome creates splendid illumination on the north side. As always, the light of the Sun was a signal of the favorable presence of the divinity. The Vestibule must have been the place where the religious ceremonies linked to the Imperial Cult took place, which were a prelude to the even more elaborate ones of the Byzantine world.

Diocletian was one of the last great Roman emperors: he tried to revive the fortunes of the Roman Empire which in the 3rd century AD was going through a serious economic and political crisis, at the mercy of opposing factions of emperor-usurpers, at the head of their respective armies.
To ensure a stable and long-lasting succession to imperial power, Diocletian devised the Tetrarchy system with two Augusti (Diocletian and Maximian) and two Caesars (Galerius and Constantius) being the designated heirs.

Diocletian was Augustus major, the more important of the two: he could also issue laws for all the others, as well as reigning over the richest part of the Empire. The emperor was a god: Dominus et Deus, rector orbis ac dominus, fundator pacis aeternaem, providentissimo princeps, that is, «Lord and god, ruler of the world and lord, founder of eternal peace and most providential prince».

«Founder of eternal peace» refers to one of the most important tasks of the Roman emperor, maintaining the Pax Deorum, that is, a good relationship with the deities so that they protect the Empire.
Two significant names were added to the name of Augustus: Jovius referring to Jupiter for Diocletian, and Erculius referring to Heracles (or Hercules) for Maximian. Near the Mausoleum of Spalato there was a Temple of Jupiter, transformed into the current Baptistery: it is possible that it was related to the Jovius attribute of the emperor, and therefore to the Imperial cult.

In 274 AD Emperor Aurelian once again introduced the cult of Sol Invictus to Rome, which became the State Cult. During the days of the Winter Solstice the celebration of Dies Natalis Solis Invicti definitively replaced that of Saturnalia. The Sun god became Dominus Populi Roman (Lord of the Roman People), and the emperor Aurelian Dominus et Deus (Lord and god), deified already in life as a descendant of the Sun god by birth.

Diocletian was devoted to Sol Invictus, and adopted the symbols of royalty created by Aurelian, such as the diadem and the cloak studded with precious stones; he was depicted with the nimbus, a kind of halo. Everyone had to prostrate before him.
The illuminations (hierophanies) were a manifestation of the divinity of the emperor/Sun god; the sacredness of his power was also extended to his Court (Sacrum Concistorium) and to the Palace itself (Sacrum Palatium): a divine status that placed them above everything and everyone.

The illuminations (hierophanies) that take place in the Vestibule during the days of the Winter Solstice (21 December) are particularly significant, because Diocletian was born on 22 December: therefore the Dies Natalis of the Sol Invictus and of the god-emperor coincided, proving that he was divine. He was "bleached" by the light that illuminated him like a theatrical spotlight.

The hierophanies of the Summer Solstice were equally important, because on that date the emperor Heliogabalus had instituted processions in honor of the Sol Invictus and Jupiter: therefore once again there is a reference to Jovius and the Imperial Cult, as well as to the maximum luxuriance of Nature linked to the cycle of the Seasons.

The hierophanies of the Equinox must have had a particular meaning, but in the Roman calendar there were no correspondences with specific holidays, as in the case of the Solstices. A reference may be that the Spring Equinox coincides with the date of the conception of Diocletian, who was born on December 22nd; the same thing had happened with Augustus, who was born on 23 September (Equinox) and therefore was conceived on 21 December (Winter Solstice) which is why he was associated with the sign of Capricorn, which appears for example in the Gemma Augustea, which recalls his divinization.

The divinity of the god-emperor and the tetrarchs was therefore confirmed and underlined by the hierophanies which were a divine signal. The power of the Augusti was made sacred by their divine descent from Sol Invictus, with whom they identified. Being gods, they were worshiped by the people who owed them absolute obedience.

There was therefore a "political" aspect linked to the propaganda of the imperial power: the persecutions of Christians were caused precisely by their refusal to recognize the divinity of the emperor, because it implicitly undermined his authority and power.
The hierophanies and the identification with the Sun ultimately served the sacred legitimation of power and imperial succession: the two designated successor Caesars were divine, a step higher than other aspirants to the throne; something which at least in theory should have prevented the armies from choosing and appointing other usurper-emperors in constant conflict with each other.

(Extract from the article published in the Proceedings of the 25th Alssa Seminar).