Luminous and shadowy bodies; the sun's size vs Epicurus
A diagram of light, shadow and rays, two suspended pyramids, and the column-shadow argument for the sun's magnitude
At the upper right a red-chalk diagram of a lit structure with rays illustrates a pen caption naming a luminous body, a shadowy body and luminous rays. A red-chalk mechanical note on two pyramids suspended horizontally and vertically explains that a small appendage a n bears the excess weight the pyramid's point gains when it is not hung at its accidental centre of gravity, weight that would otherwise fall upon the other pyramid. A longer passage argues against Epicurus: seeing column-shadows on opposite walls equal to the column's diameter led him to judge the sun no thicker than the column, missing that the shadow's diminution is imperceptible over the sun's great distance; were the sun smaller than the earth, much of our hemisphere's stars would go dark.
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Luminous body, shadowy body and luminous rays
A pen caption beside the upper diagram labels the three elements of a shadow-casting scheme: the luminous body (light source), the shadowy body that intercepts it, and the luminous rays connecting them. The red-chalk sketch of a lit structure with radiating lines gives the geometry to which the caption refers.
Column-shadows and the sun's true magnitude
Leonardo suggests Epicurus saw the shadows of columns cast on opposite walls equal to the column's diameter, and, taking the shadow as parallel from start to finish, judged the sun to be of the same breadth and so no thicker than the column, not perceiving that the shadow's diminution is imperceptible over the sun's great distance. He adds that were the sun smaller than the earth, the stars of much of our hemisphere would be without light, against Epicurus's claim that the sun is as great as it appears.
Two suspended pyramids and the appendage a n
The little appendage a n sustains the excess weight that the pyramid's point acquires when the whole is not suspended at the centre of its accidental gravity. Without a n to bear it, that excess would discharge upon the other pyramid and fall at the true centre of natural gravity of the pyramid set upright.