When you think of Leonardo Da Vinci, you probably think of the Mona Lisa or 16th-century submarines or, maybe, a certain suspenseful novel that has been made into a movie. That's old school. From now on, think of the Moon. Little-known to most, one of Leonardo's finest works is not a painting or an invention, but rather something from astronomy: He solved the ancient riddle of Earthshine.
You can see Earthshine whenever there's a crescent Moon on the horizon at sunset. Look between the horns of the crescent for a ghostly image of the full Moon. That's Earthshine. There should be one on July 15 – and at dawn that day it will also be framed in a celestial triangle with Jupiter and Venus. Worth getting up early to see it – hope there are no clouds!
For thousands of years, humans marveled at the beauty of this "ashen glow," or "the old Moon in the new Moon's arms." But what was it? No one knew until the 16th century when Leonardo figured it out.
In modern times, the answer must seem obvious. When the sun sets on the Moon, it gets dark--but not completely dark. There's still a source of light in the sky: Earth. Our own planet lights up the lunar night 50 times brighter than a full Moon, producing the ashen glow.
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.)
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.
To Leonardo, Earthshine was an appealing riddle. As an artist, he was keenly interested in light and shadow. As a mathematician and engineer, he was fond of geometry. All that remained was a trip to the Moon. This marvelous Renaissance man made that mental journey.
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.
Not oceans – clouds do most of Earth’s reflecting. And the Moon has no atmosphere. But he was basically right. Nice going, Leonardo da Vinci!