The Leonardo Da Vinci Glow

Five hundred years ago, Leonardo Da Vinci solved an ancient astronomical riddle: the mystery of Earthshine.

When you think of Leonardo Da Vinci, you probably think of the Mona Lisa or 16th-century submarines or, maybe, a that certain suspenseful novel - The Da Vinci Code. That's old school. From now on, think of the Moon.

Little-known to most, one of Leonardo Da Vinci'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.

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.

Photo credit: Andy Skinner. A crescent moon with Earthshine over Yosemite National Park in October 2004.

In 2005, post-Apollo, 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 armoured 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. It was a mental journey: