EARTH SHINE
When you
think of Leonardo Da Vinci ,you probably think of the Mona Lisa or 16th –century submarines or, maybe ,a certain popular novel .That’s now 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 cresent Moon on the horizon at sunset. Thursday, Oct 6, is a good night: sky map .Look between the horns of the cresent 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 o9ld Moon in the new Moon’s arms .But what was it? No one knew until the 16th century when Leonardo figured it out.
In 2005, post-Apollo, the answer must seem obvious .When the
sun sets on the Mo9on, it gets
dark—but not completely dark .There’s still a source of light in the sky: Earth .Our own planets 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, scub a 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 it 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: 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
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