"In truth, no one knows the Pantheon's exact age. One legend says that the first Roman citizens built the original Pantheon on the very site where the current one still stands in the Campo Marzo - modern Rome's business district. The ancients constructed this first Pantheon after Romulus (753-716 BC), their mythological founder, ascended to heaven from that site. They dedicated it to Romulus and some of his divine ancestors and, for centuries, held rites and processions there.
Most historians, however, claim that Marcus Vipsanius Agrippa built the first Pantheon in 27 BC, a rectilinear, T-shaped structure, 144 feet by 66 feet (44m x 20m), with masonry walls and a pitched timber roof. It burned in the great fire of 80 AD, was rebuilt by Emperor Domitian, but was struck by lightening and burned again in 110 AD. By 120 AD, Hadrian began designing a Pantheon reminiscent of Greek temples and far more elaborate than anything Rome had yet seen. His plans called for a structure with three main parts: a pronaos or entrance portico, a circular domed rotunda or vault, and a connection between the two. The rotunda's internal geometry would create a perfect sphere, since the height of the rotunda to the top of its dome would match its diameter: 142 feet (43.30 m). At its top, the dome would have an oculus or eye, a circular opening, with a diameter of 27 feet (8.2m), as its only light source.
Hadrian said, "My intentions had been that this sanctuary of All Gods should reproduce the likeness of the terrestrial globe and of the stellar sphere...The cupola...revealed the sky through a great hole at the center, showing alternately dark and blue. This temple, both open and mysteriously enclosed, was conceived as a solar quadrant. The hours would make their round on that caissoned ceiling so carefully polished by Greek artisans; the disk of daylight would rest suspended there like a shield of gold; rain would form its clear pool on the pavement below, prayers would rise like smoke toward that void where we place the gods." - Monolithic.com