The Da Vinci Glow

http://science.nasa.gov/headlines/y2...t_leonardo.htm

The Da Vinci Glow
NASA Science News
October 4, 2005

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


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. Thursday, Oct. 6, is a good night: sky map
http://science.nasa.gov/headlines/y2...th_06oct05.gif
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.

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:

see captionIn 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.

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. Clouds are much better reflectors of sunlight than water. 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.

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:

"Leonardo was here."

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:

Why did you say that the reflection was 50 times brither than the sun?

"DarkD" writes:

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:

Why did you say that the reflection was 50 times brither than the sun?

That's not what he said - he said that the earth lights up the moon 50x
brighter than the full moon appears to us on Earth. That's a bit of a
difference from what you're claiming.

--
Patrick "The Chief Instigator" Humphrey ) Houston, Texas
chiefinstigator.us.tt/aeros.php (TCI's 2005-06 Houston Aeros)
LAST GAME: Chicago 5, Houston 3 (April 26)
NEXT GAME: Friday, October 7 vs. San Antonio, 7:35

"The Chief Instigator" wrote in message
...
"DarkD" writes:

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:

Why did you say that the reflection was 50 times brither than the sun?

That's not what he said - he said that the earth lights up the moon 50x
brighter than the full moon appears to us on Earth. That's a bit of a
difference from what you're claiming.

Sorry, I meant the moon.
So the glow is 50 times brighter than when its a full moon? Is this the time
when you see the moon glowing really really yellow?

In article ,
DarkD wrote:

"The Chief Instigator" wrote in message
...
"DarkD" writes:

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:

Why did you say that the reflection was 50 times brither than the sun?

That's not what he said - he said that the earth lights up the moon 50x
brighter than the full moon appears to us on Earth. That's a bit of a
difference from what you're claiming.

Sorry, I meant the moon.
So the glow is 50 times brighter than when its a full moon? Is this the time
when you see the moon glowing really really yellow?

No - what he said is this: as seen from the Moon a "full Earth" shines
some 50 times brighter than a full Moon shines as seen from the Earth.

Which also means that out in space, far enough away to make the Earth and
the Moon appear full side-by-side in the sky, the Earth will shine some
50 times brighter than the Moon.

--
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