Why was the pantheon constructed with a 30ft diameter oculus in its roof?

Answer 1

The empty circle in the centre of the dome of the Pantheon, the oculus (eye) has two purposes. One is to let the light in. There are only two sources of light in the rotunda of the Pantheon: the entrance and the oculus, The other is to contribute to decreasing the weight of the dome. It is still the largest unreinforced concrete dome in the world. Nowadays there are domes which are bigger than that of the Pantheon. However, they are made of reinforced concrete. To manage to make such a large dome (its diameter is 43.3 metres, 142 ft.) with unreinforced concrete, its weight needed to kept to a minimum. If fact, this gigantic structure amazingly weighs only five tons.

To achieve this, different types of aggregate were used. Roman concrete was made by mixing lime with a volcanic rock called pozzolana. This created a binder to which an aggregate was added to give it mass. The dome of the pantheon is 6.4 metres (21 ft.) thick at the bottom and crushed travertine rock was used as an aggregate. As it goes upwards, the dome becomes thinner and is 1.2 metres (3.9 ft.) thick at the top. Besides getting thinner, the aggregate changes. In the mid-section of the dome it is crushed terracotta tiles, which are lighter. In the top section it is made of crushed tuff and pomice, which are porous stones which are even lighter. Finally, the oculus makes the apex weightless. It has been calculated at without these weight-saving techniques the stresses on dome would be 80% higher and it would collapse under its weight. Roan concrete was a though as modern concrete, but it was less fluid and had to be layered by hand.

Answer 2

It allows light into the building by day, and as hot air rises, cools the building in summer.allows rain to enter and fall to the floor, where it is carried away through drains.