The moon as a polished mirror and the light of the earth
Why the sun's image on a smooth moon is small, and how earthshine would work
Leonardo tests whether the moon is a smooth, spherical mirror. If it were polished and spherical, the sun's image on it would be intensely bright but confined to a small part of its surface, just as a ball of burnished gold set in darkness with a distant light shows the light's image only on a small patch while the rest mirrors the surrounding darkness and stays invisible to a distant eye. He argues the same would happen on a smooth, lustrous, dense moon like other mirroring bodies. A marginal note urges the reader to prove that, seen from the moon or a star, our earth would serve the sun as the moon does for us, and that the sun's image on the sea cannot appear as a single sun the way it does in a flat mirror. The folio bears the Bibliotheque de l'Institut stamp.
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The sun's image on a smooth moon would be small and bright
If the moon is smooth and spherical, the image of the sun in it is intensely luminous yet fills only a small part of its surface. The same would occur on a moon that were smooth, lustrous and dense, like the bodies that mirror.
The burnished-gold ball proof
Take a ball of burnished gold in darkness with a distant light: though the light illuminates about half the ball, a distant eye sees the image only on a small part while all the rest mirrors the surrounding darkness and stays invisible.
Earth would serve the sun as the moon does for us
Prove that, if you stood on the moon or on a star, our earth would appear to perform the same office with the sun that the moon performs for us.
The sun's image on the sea is not a single sun
Prove how the image of the sun on the sea cannot appear as a single sun, the way it appears in a flat mirror.