Galileo: Circa la figura, sito e grandezza dell' Inferno di Dante

Coming then to an explanation of Manetti's opinion, and first of all as far as the shape is concerned, I say that this is the form of a concave surface called conic, the peak of which is at the center of the world, with the base towards the surface of the earth. But then, let us abbreviate and simplify the reasoning. Linking shape, position, and size, let us imagine a straight line stretching from the center of the earth (which is still the center of gravity and of the universe) as far as Jerusalem, and an arc which extends from Jerusalem above the surface of the water and earth for a twelfth of its major circumference. Such an arc will terminate with one of its extremeties in Jerusalem. If, from the other end, another straight line were to be drawn as far as the center of the world, we would have the sector of a circle, contained by the two lines coming from the center and from the said arc. Let us then imagine that, the line joining Jerusalem and the center being immobile, the arc and the other line are set in motion, and that with such a movement it slices the earth, and keeps moving until it returns to its starting point. A part similar to a cone would be cut out of the earth, and if we imagine this to be removed, there would be left, in its place, a hole in the form of a cone; and this is Hell.

Galileo Galilei, from Due lezioni all'Accademia Fiorentina circa la figura, sito e grandezza dell'Inferno di Dante.