And you were there as long as I descended; but when I turned, that’s when you passed the point to which, from every part, all weights are drawn.Inferno XXXIV, 109-111
Dante and Virgil have been climbing down in the same direction on Lucifer’s hairy body. A moment later, Virgil turns around to climb up toward the exit from Hell. Asking his master for an explanation, the disoriented Poet receives a lesson on the center of gravity. The two men, Virgil tells him, have changed direction when reaching the center of the Earth, the point where all weights converge. The question of the center of gravity, as distinct from the geometric center, was examined by medieval philosophers—and by Dante himself in the Questio de aqua et terra—to explain the emersion of a part of the Earth from the elemental sphere of Water.