The Da Vinci Glow

October 4, 2005: 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'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.

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

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
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 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. It was a 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.

Right:
Leonardo made this sketch of a crescent moon with Earthshine.
It appears in the Codex Leicester. [More]

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.

Above:
A picture of Earth taken by Apollo 11 astronauts. From the
Moon, Earth is 4 times wider than the sun and about 50 times
brighter than a full Moon. [More]

In
the decades ahead, humans are going to travel in person where
Leonardo's imagination went 500 years ago. NASA plans to send
astronauts back to the Moon no later than the year 2018. Unlike
Apollo astronauts, who stayed for a few days at most, these
new explorers will remain on the Moon for weeks and months.
In the process, they'll experience something Apollo astronauts
never did: nightfall. A lunar "day" is 29.5 Earth-days
long: about 15 Earth-days of light, followed by 15 Earth-days
of darkness. Apollo astronauts always landed in daylight and
took off again before sunset. Because of the bright sun, they
never saw the soft glow of Earthshine at their feet. But the
next generation of astronauts will.

And
just maybe, on a late-night stroll behind the outpost, guided
by the soft light of Earth, one of them will bend over and
scratch something in the moondust: