As we will see further on, some scholars already conjectured about an astronomical meaning for Roccabruna; nobody ever thought about an astronomical orientation of the Accademia, which is our significant discovery.
As we said, the Accademia had a series of see-through rooms, aligned along a main longitudinal axis, oriented from north-west to south-east (see plan figure 12). This axis coincides with the ideal line joining the point where the Sun is rising at dawn during the days of Winter Solstice (at south east) and the point where the Sun sets on Summer Solstice (at north-west).
The series of ‘see through’ rooms was designed so that a man standing at the center of any of its axial doors, during the days of Winter Solstice can still see the light entering at dawn from the window of room AC89, then passing through the doors of the Zooteca AC88, of the Temple of Apollo AC78 and of the two courts AC60 and AC41 that were preceding it on the north.
On the other hand, during the says of Summer Solstice, an observer standing in room AC89 can see the Sun set on the opposite side, with its rays passing through the series of axial doors, in reversed order.
Fulcrum of these light phenomena is the Temple of Apollo, the most monumental hall of the complex (figure 13).
Its name - Temple of Apollo or Tempio delle Muse et di Apolline - was created in the XVI century by Pirro Ligorio, the first who studied and excavated Villa Adriana on a large scale1. The name originates from the number 10 of the niches for statues on its upper floor: nine Muses plus Apollo makes ten. The name is still in use, like the imaginative ‘Zooteca’, which was created by Ligorio for the nearby garden with porch.
The Temple of Apollo is circular, with a diameter of 13,85 meters (about 46 roman feet2). Its lower floor is subdivided in twenty panels (thirteen are still standing) which still have fragments of plaster with traces of fresco: white panels with a red frame were alternating with red panels framed by a white border. The small columns of the lower floor supported an architrave made of bricks, on top of which is a masonry ‘drum’ and the upper floor (see further on figure 15).
Also the upper floor was divided in twenty sectors: ten windows were alternating with ten niches. Seven windows and six niches are still standing, and in the niches there probably were statues; their ceilings still show fragments of black fresco with a white border.
As far as the roofing of the Temple of Apollo is concerned, antiquarian scholars always thought it was a dome. Francesco di Giorgio Martini, author of the most ancient sketch of the Villa that we have (about 1465), drew an umbrella-like dome3 (figure 14). The subsequent antiquarian scholars, Pirro Ligorio, Francesco Contini, Giovan Battista Piranesi, Agostino Penna e Luigi Canina agreed with his reconstruction, as did Heinz Kähler4 and also Italo Gismondi in his plastic model.
We drew a 3D reconstruction of the Temple of Apollo with a dome and central oculus (figure 15), whose dimensions were estimated following the same proportion that can be observed in the Pantheon of Rome between the diameter of the dome and that of the oculus.
On december 19th, 2009 (Winter Solstice) the archaeoastronomer Giuseppe Veneziano photographed the light phenomena that occur at dawn: as expected, the Sun raises at the center of the window of room AC89 (see plan figure 13) and goes through the series of axial rooms. In the picture (figure 16) the Sun is seen from room AC88, the so called Zooteca, and is visible from all the doors of the axial see-through rooms.
On june 20th, 2010 (Summer Solstice) the light phenomena were more complex than what we expected. On june 11th, 2006 - about ten days before Solstice - I had photographed just one light spot on one of the panels of the Temple of Apollo. But on june 20th, 2010, the light spots became two.
At the beginning of our observations, the Sun as expected entered through one of the windows of the upper floor (figure 17), illuminating the last preserved panel of the lower floor, exactly as had happened in 2006. But few minutes later appeared a second spot of light, slimmer than the previous one, which I had not seen in 2006 (figure 18).
This happens only during the days of Summer Solstice, when the sun is setting at the extreme north-western point of its course during the whole year. During the rest of the year, its rays do not reach that window. The two light spots started to move gradually from left to right, with an arched movement.
When the Sun sets there is just one light spot. The Sun enters through the north-western door of the Temple of Apollo (figure 19) and illuminates the door between the Zooteca AC88 on the opposite side, and the axial room AC89, reaching its outer window. The direction of the light is reversed compared to what we saw on Winter Solstice (figure 20).
Unfortunately, just the north-eastern half of the Temple of Apollo is still standing, but it is likely that also during Winter Solstice there was a double spot of light on the panels; this could be verified with a 3D model or a computer simulation.
If we accept the hypothesis that the Temple of Apollo had a dome with a central oculus, we can try to calculate other light phenomena produced though this central opening in certain days of the year (figure 21).