The best thing about today's APOD is the caption's "Leonardo da Vinci
Visualizing this in the 1500s required a wild kind of imagination. No one had ever been to the Moon and looked "up" at Earth. Most people didn't even know that Earth orbited the sun. (Copernicus' sun-centered theory of the solar system wasn't published until 1543, twenty-four years after Leonardo died.)
Imagine. One wonders what Leonardo thought about the Earth's place in the Universe.
Wild imagination was one thing Leonardo had in abundance. His notebooks are filled with sketches of flying machines, army tanks, scuba gear and other fantastic devices centuries ahead of their time. He even designed a robot: an armored knight that could sit up, wave its arms, and move its head while opening and closing an anatomically correct jaw.
In the inset at left is Leonardo's sketch of "the ghostly glow",
or Earthshine, circa 1510, from the Codex Leicester.
In Leonardo's Codex Leicester, circa 1510, there is a page entitled "Of the Moon: No Solid Body is Lighter than Air." He states his belief that the Moon has an atmosphere and oceans. The Moon was a fine reflector of light, Leonardo believed, because it was covered with so much water. As for the "ghostly glow," he explained, that was due to sunlight bouncing off Earth's oceans and, in turn, hitting the Moon.
The Earth from Apollo 17.
He was wrong about two things:
First, the Moon has no oceans. When Apollo 11 astronauts landed at the Sea of Tranquility, they stepped out onto rock. Lunar "seas" are made of ancient hardened lava, not water.
Second, Earth's oceans are not the primary source of Earthshine. Clouds are. Earth shines because it reflects sunlight, and clouds do most of the reflecting. When Apollo astronauts looked at Earth, the oceans were dark and the clouds were bright.
But these are quibbles. Leonardo understood the basics well enough.