Five hundred years ago, Leonardo Da Vinci
solved an ancient astronomical riddle: the mystery of Earthshine.
by Dr Tony Phillips
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.
moon with Earthshine over Yosemite National Park in October
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
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
made this sketch of a crescent moon with Earthshine. It
appears in the Codex Leicester.
He was wrong about
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.
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.
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.
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