Thanks to the cooperation with the local scholar Zlatko Andrijašević, we discovered archaeoastronomy in Diocletian's Palace in Split (Croatia) and photographed extraordinary illuminations inside the Vestibule, during the Solstices and the Equinoxes.

The large oculus of the Vestibule functions like the one of the Pantheon in Rome: it creates circles of light at different heights according to the seasons, as explained in our book «Pantheon Architecture and Light» dedicated to the Arc of Light.

Like Hadrian in the Pantheon, Diocletian could claim to be so powerful to command the course of the Sun inside the Vestibule, during the illuminations. He was presiding important religious rituals, from a terrace in front of the Vestibule overlooking the Peristyle.

The illuminations were a sign of the presence and approval of the divinities. Diocletian was already divine in life, and therefore one step higher than any rivals in the succession; this legitimated his power.
Acclaimed emperor in 284 AD, he is sadly known for the persecutions against Christians, which arose from their refusal to consider the emperor as a god, dangerously undermining his authority.

In 274 AD with the emperor Aurelian the cult of Sol Invictus became a state cult and the emperor was proclaimed Dominus et Deus, descendant of the sun god: as such he wore the diadem and a cloak embroidered with precious stones.
Diocletian and Maximian used those same symbols of power. Diocletian wore the diadem, and proclaimed himself «rector orbis ac domino, fundator pacis aeternam, providentissimo princeps», which means «ruler of the world, founder of eternal peace, and provident prince».

Actually he was one of the last great emperors, who tried to save the Roman Empire from disintegration and economic crisis, with the important Price Edict. In an era of endless civil wars between competing generals - it was enough to have an army to be acclaimed emperor - he tried to ensure a stable and lasting succession to power, creating the Tetrarchy with two emperors - the Augusti - and two designated successors - the Caesars, and divided the empire into four parts.

The Augusti were Diocletian, who ruled the East East (Egypt, Libya and Asia) and Maximian, who ruled Italy, Rezia, Sicily, Sardinia and Africa.
Instead, the Caesars were Galerius, who ruled Illyria, Macedonia, Greece and Crete, and Constantius, who ruled Gaul and Britain.

In 303 AD. Diocletian and Maximian celebrated the Triumph of the Persians in Rome, with a spectacular entrance on a chariot pulled by four elephants. Immediately afterwards Diocletian abdicated and retired to his Palace in Split, which he had begun to build in 295 AD, and where he said he preferred the cultivation of his cabbages to the exercise of power.