Why planets look larger at the horizon
Curved elemental spheres and the apparent size of celestial bodies
Leonardo poses a problem of celestial optics: why do the planets appear larger in the east than overhead, when the reverse should be true, since a body at the zenith is some 3500 miles closer than one at the horizon? His answer invokes the curved layers of the elements through which the 'species' (images) of celestial bodies pass to the eye: the central ray strikes these curved layers at unequal angles, and the path is longer at the horizon, as the excess of a b over a d shows. A diagram of concentric elemental and celestial spheres carries the labels d and a b c.
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Why planets look larger at the horizon
Leonardo asks why the planets appear larger in the east than above us, when the contrary should hold, since at mid-sky they are about 3500 miles closer to us than at the horizon.
Curved elemental spheres bend the celestial species
All the layers of the elements through which the species of celestial bodies pass to the eye are curved, and the angles at which the central line penetrates them are unequal, and the distance is greater, as the excess a b over a d shows; by the ninth of the seventh the magnitude at the horizon is proved.