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Al Stahler: Seeing colors

Al Stahler
Columnist

KNOW & GO

WHO: Nevada County Astronomers

WHAT: Talk/slide show on Astrobiology

WHEN: 7 p.m. Wednesday

WHERE: Madelyn Helling Library, 980 Helling Way, Nevada City

For some months, planet Venus has shone, after sunset, bright in the west. Most nights, Venus shines alone, but, once a month, the crescent moon sidles up, joining Venus for an evening.

Like Earth, the moon turns, in rotisserie-mode, beneath the sun. So, like Earth, all parts of the moon enjoy both day and night. How much of the sun-lit side of the moon we see depends on its phase — crescent, quarter or full.

The part of the moon not lit by the sun should, by rights, be — well — dark as night. But look closely tonight, and notice that — for a few hours after nightfall — the dark, night-side of the moon is lit by a ghostly light.

After the sun, the moon is the brightest object in our sky. Were we standing on the moon, though, we’d see Earth shining way more brightly than the moon ever could.

Looking at the moon, we see sunlight reflecting off dark rock. Looking at Earth, we see — mostly — sunlight reflecting off bright clouds.

The ghostly light on the night side of the moon is sunlight, but not coming directly from the sun. Rather, it’s sunlight that has first struck Earth, then reflected back out into space. Most reflects off clouds — after sunset, off clouds over the Pacific. Some of that reflected light strikes the moon, lighting the night-side of the moon … with earthshine.

Right now, the crescent moon is growing, night-to-night, toward full. Two weeks from now, after full moon, the crescent moon — shrinking back toward new — will rise before dawn. The night-side of the moon will again be lit by earthshine — but now, reflecting off clouds over the Atlantic.

A large fraction of Earth is always covered with cloud. As years go by, could the cloudy fraction of the Earth be growing larger? Or smaller? Or staying the same?

In an attempt to find an answer, climatically-curious astronomers are keeping track of earthshine, to see if it’s slowly getting brighter, dimmer or neither.

Hold a CD in the sun, and bazillions of tiny pits in the plastic break sunlight into the colors of the rainbow: red-orange-yellow-green-blue-indigo-violet (ROY G. BIV).

Just beyond violet is another color — ultraviolet — but our eyes cannot see it.

And just below red is another color — infrared (IR) — but, again, our eyes cannot see it.

Suppose we could see IR — at least, the IR closest to the red we can see: near-IR (NIR). (We can’t see IR even farther from red, either, but we can feel far IR as heat.).

Suppose we could see NIR. Every time we clicked the TV remote, we’d see its tip, glowing in NIR. But we’d also see something stranger than that.

Plants soak up pretty much all the colors of the rainbow, except for a bit of green, which they reflect, which is why plants look green. But just outside the visible rainbow, plants reflect a lot of NIR – way more than green. If we could somehow see NIR, we wouldn’t see plants as green, we’d see them as bright, bright NIR.

Astronomers studying earthshine believe they have seen NIR from plants in Earth’s reflection on the moon.

Might it be possible to detect plants, growing on planets circling other stars, by looking for NIR or some other color these extraterrestrial plants reflect back out into space?

At the next meeting of Nevada County Astronomers, 7 p.m., Wednesday at the Madelyn Helling Library, I’ll present a talk/slide show on astrobiology – looking for life on other worlds. It’s free, and all ages are welcome.

Al Stahler enjoys sharing science and nature with friends and neighbors on KVMR (89.5FM), and can be reached at stahler@kvmr.org.


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