At midday on the Spring Equinox, light streams in through the twenty-seven foot diameter oculus of the Pantheon and aligns perfectly with the doorway.
Scholars Robert Hannah and Giulio Magli compare the design of the Pantheon with that of a hemicyclium, a type of sundial formed from a stone block with a single hole allowing in light. The interior was carved with parabolic lines to aid with telling-time. Hannah and Magli do not believe that the architects working first for Trajan and then for Hadrian intended the focus of the “temple” to be a time-keeper but rather note that the association with the sun was symbolic and showed the Emperor as “’keeper’ of the regular course of time.” (510)
The connection between the Emperor and the sun had a long history by the time the Pantheon was being rebuilt. The Romans saw connections between the Emperor’s apotheosis and heavenly bodies during the reign of Augustus. The passage of the sun also influenced the placement of the Ara Pacis and obelisk in the Campus Martius and the design of the octagonal room in the Domus Aurea. From the date of Nero’s accession on October 13, 54 CE the sun “ascends” up the interior walls until the winter solstice. The scholars conclude that it was the movement of the sun through the interior that may explain the famous statement that the room “rotated.” (501-2)
Finally, Hannah and Magli note that the Pantheon should also be understood within the wider symbolic geography of the ancient city explaining that on the summer solstice (when looking from the porch of the Pantheon) the sun appears to set over the other great monument built in the ancient city during the reign of Hadrian, his Mausoleum. Indeed, this position, which required the building of a new bridge, connected the Mausoleum to both the Pantheon along the solstice sunset line and the Ara Pacis along the same East-West parallel. They explain that, “sun and time, then, were linked architecturally into cosmological signposts for those Romans who could read such things.” (507)
References: Hannah, Robert and Giulio Magli. “The Role of the Sun in the Pantheon’s Design and Meaning.” Numen. Vol.58. No.4 (2011): 486-513; Hannah, Robert. “The Pantheon as Timekeeper.”
Pantheon, Exterior and Interior (118-128 CE) (Jennifer D. Webb)
“Pantheon Birthday of Rome” (Geografia Sacra)
Further reading: Tod A. Marder and Mark Wilson Jones, The Pantheon: From Antiquity to the Present. Cambridge: Cambridge University Press, 2015; Penelope J. E. Davis, Death and the Emperor: Roman Imperial Funerary Monuments from Augustus to Marcus Aurelius. Cambridge: Cambridge University Press, 2000.