Gravity

Climbing Lucifer in Dante's Inferno

A long time ago (it seems) I touched on gravity and said "there was no working theory of gravity yet; just a feeling that substances could be heavier or lighter." Theories of gravity did not need Newton to become useful to understanding the world.

Still, if the inhabitants of this globe knew it was a globe, they had to come to terms with how things stayed on it. John Mandeville wrote a book of travels in the 14th century in which he addresses this issue:

from what part of the earth that men dwell, either above or beneath, it seemeth always to them that dwell that they go more right than any other folk. And right as it seemeth to us that they be under us, right so it seemeth to them that we be under them. [Mandeville, Voyage and Travels, chapter xx]

The Middle Ages understood that objects inclined toward the earth itself, wherever they were upon its spherical surface. In fact, gravity was sometimes described as "kindly inclining."

Dante (1265-1321) in his Inferno has his traveler and guide (Virgil) descend through the levels of Hell—not a metaphorical journey, but descending deeper and deeper below the surface of the Earth—finally reaching the gigantic body of Lucifer embedded in the ice. They start climbing down his body when, at his groin region (the bottom-most part of the world), down becomes up, and they have to reverse their orientation. They are now climbing up to his feet.

Was Dante just speaking metaphorically? Did he understand gravity as it operates at the center of the Earth?

Maybe he did. Vincent of Beauvais (c.1190 - 1264) wrote an encyclopedia that was very popular in the Middle Ages. It is not unexpected that Dante would have been very familiar with it. Vincent was well aware of the spheroid nature of the Earth, and that objects fell directly to Earth no matter where they were on the surface. In his Speculum Naturale ["Mirror of Nature"], he explains as much of the world as he can in 3,718 chapters spread across 32 books. In Book vi, he tells the curious what would happen to a stone dropped into a hole that goes straight through the globe to the other side. He says that it would not fight gravity and rise to the other side, but would stay at the center.

The acceptance of a terrestrial globe forced scholars and philosophers to re-think the action of falling bodies and their relationship to the Earth. Common sense led them to arrive at the proper effects of gravity, even if they did not have the science or mathematics to understand the cause of gravity.