Astronomy
The Moon, the Sun, and the Blue of the Sky
The moon has no light of itself but so much of it as the sun illumines, of so much do we see.
— Leonardo da Vinci
Overview
Leonardo's astronomical observations are scattered across several manuscripts, particularly the Codex Leicester. He was fascinated by the light of the moon — why it shines, why it appears to change, and why Earthshine (the faint glow on the dark part of a crescent moon) exists. He correctly deduced that the moon reflects sunlight and that Earthshine is sunlight reflected from Earth onto the moon and back to us.
He also investigated the apparent size of the sun at different positions in the sky, the reason for the sky's blue color (anticipating Rayleigh scattering by four centuries), and the nature of stars.
The Light of the Moon
Reflected sunlight and Earthshine
The moon has no light of itself but so much of it as the sun illumines, of so much do we see. And its night receives just as much brightness as is lent it by our waters as they reflect the image of the sun, which is mirrored in all those waters which are on the side nearest the sun. The outside or surface of the waters forming the seas of the moon and of the seas of our globe is always ruffled little or much, or more or less — and this roughness causes an extension of the numberless images of the sun which are repeated in the ridges and hollows and sides and fronts of the innumerable waves; that is to say in as many different spots on each wave as our eyes find different positions to view them from.
This passage about the moon's light shows Leonardo at his most methodical. He breaks the problem into components: the moon reflects sunlight; Earth's oceans reflect sunlight to the moon (Earthshine); rough water surfaces multiply the reflections. Each step follows logically from the last. -D
Why the Sky Is Blue
Light, atmosphere, and the darkness of space
I say that the blue which is seen in the atmosphere is not its own colour, but is caused by the heated moisture having evaporated into the most minute and imperceptible particles, which the beams of the solar rays attract and cause to seem luminous against the deep, intense darkness of the region of fire that forms a covering above them.
Not quite right — it's not moisture particles but air molecules that scatter blue light preferentially (Rayleigh scattering, described in 1871). But the structure of his reasoning is correct: the blue is not the atmosphere's own color, it's caused by particles being illuminated against the darkness beyond. He got the mechanism wrong but the logic right — 350 years before the physics existed to explain it properly. -D
Related Subjects
Related Topics
- Optics
- Physical Geography
- The Nature of Water
- Color
- Inventions — Telescope concept, heliocentric intuition, Earthshine
Leonardo's astronomy is remarkable not for telescopic observation — he predated the telescope by a century — but for the quality of his reasoning. He figured out Earthshine before Galileo was born. He proposed that the sky is blue because of sunlight interacting with moisture particles in the atmosphere — not exactly right, but the logic is sound and the method is impeccable. He asked the right questions and reasoned from first principles. -D