Rays past the moon, and why a distant light looks round
Eclipse-ray geometry, with a flame as a model for distant luminous bodies
Two ringed diagrams at the top plot the orbits of the sun and moon around the earth, where Leonardo considers the case in which solar rays grazing the moon's edges meet, at a right angle, rays reflected from the earth. A second note, illustrated by a sketch of flames, asks why any elongated luminous body appears round at a great distance. He answers that, like a lead die flattened by hammering, the light spreads equally in all directions until its original core becomes negligible, so it looks round — proof that a star's 'horns' cannot be perceived from afar.
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Right-angle meeting of solar and earth-reflected rays at the moon
The upper diagrams trace the orbits of the sun and moon about the earth. Leonardo considers the moment when the intersection of solar rays grazing the moon's extremes forms a right angle with the intersection of rays reflected from the earth that also graze the moon; the note breaks off before its conclusion.
Why an elongated light appears round at a distance
A luminous body of elongated shape is never perfectly round, but at great distance behaves like a hammered, flattened lead die that takes a circular form. Because the light gains width equally on every side, its original core counts for nothing against that gain, so the uniform spread makes it look round — showing that a star's horns cannot be perceived from afar.