A fluid body does not resist anything heavier than itself; a heavy body, meeting no resistance, seeks the lowest place, and within the universe there is nothing lower than its centre. Therefore the centre of a heavy body that descends freely into the centre of the world comes to rest at the centre of the world, and its opposing weights will be equal. That weight will be less heavy which descends by a less oblique line; no heavy body is held up by a support that lies outside its central line. That heavy body will have the shorter motion which descends by the less oblique line, and so conversely.

It is in the nature of a heavy body that, with every degree of nearness it gains toward the centre, it loses speed and weight. See the heavy body a, which in descending from c continually gains nearness to the centre of the world; and when this heavy body is at a, it weighs along the line a b and no longer along the oblique line a d.

That heavy body which by its motion gains speed toward the centre will make the shorter motion, the one that descends by the more oblique line. See, in the great obliquity b a, that the weight o halted at its middle between right angles, that is perpendicular above the centre, and this heavy body travelled little distance. Then see the line a g, where the weight n stopped upon that line after a longer path. The motion of that heavy body will be the shorter which descends by the more oblique line.

That heavy body moves more, and comes nearer to the centre of the world, which descends by the less oblique line. No heavy body will stay upon the support on which it rests unless the central line of its weight touches the front of that support. Here the shorter pendant makes the weight heavier. Unequal weights that resist one another equally upon the scales will not have their pendants leave the beam of the scales at equal angles to one another.

The waves of the water enlarge the image of the thing mirrored in it. It is impossible that at any distance the image of the sun formed on the surface of a spherical body should occupy half of that sphere. These are two figures; so you shall make the one apart from the other, that is, the waving water from the still water.

How the sun cannot be mirrored in the body of the moon, it being a convex mirror, in such a way that as much of it as the sun lights up, so much the moon should mirror back, unless indeed that moon had a surface fit for mirroring, one that were rough after the manner of the surface of the sea when it is partly stirred by the wind.

These waves are made along every line, after the likeness of the husk of a pine cone. Here you have to prove how the earth performs toward the moon all those same offices that the moon performs toward the earth. The moon does not shine with its reflected light as the sun does, because the light of the moon does not take up the sun's light continuously over its surface, but on the crests and hollows of the waves of its waters; and because that sun is confusedly mirrored in the moon through the mingling of the shadows that lie upon the shining waves, its light is therefore not bright and clear like the sun's.

The earth between the moon at the fifteenth (full moon) and the sun.

Here the sun is in the west and the moon in the east, at the fifteenth (full moon).

The moon between the earth (at the fifteenth, full moon) and the sun.

Here is the moon, which has the sun to the east and the earth to the west.