Friday, January 18, 2013

The Dark Side of the Moon

“I'll see you on the dark side of the moon.” Folks with a little astronomy knowledge cringe when they hear these Pink Floyd lyrics. They will patiently explain there is no dark side of the moon. The moon turns a revolution over about 4 weeks. The far side as well as the near side see two weeks of darkness as well as two weeks of sunshine.

But one side is darker. Since the moon is tide locked, the far side never sees earthlight. On the other hand, someone standing on the moon’s nearside will always see earth hovering in the same region of the sky.

Viewed from the earth’s surface, both the sun and the moon subtend about half a degree. The moon’s albedo is .12, meaning it reflects about 12% of the sunlight that hits it. The moon is nearly as dark as charcoal, it only looks bright against the black void of space when our eyes have adjusted to the night’s darkness. Even the above graphic exaggerates the moon's brightness -- the sun is about 100,000 times brighter than the moon.

Viewed from the moon’s surface, the sun subtends half a degree (just as when seen from earth). But earth subtends about 2 degrees. Moreover the earth reflects about 2.5 times more light than the moon, having an albedo of around .3.

Above is a photo taken by NASA's DSCOVR satellite as the moon passed in front of the earth. The Deep Space Climate Observatory is one million miles from the earth, lieing between the earth and sun.

The larger apparent diameter and higher albedo means the earth seen from the moon is about 34 times brighter than the moon seen from earth.

The Bright Side

Let's imagine an astronaut standing at the moon's closest point to the earth. Not far from Mösting A Crater, 0 degrees latitude, 0 degree longitude. From this location, the Earth always hovers directly overhead.

Here is our astronaut pointing his iPhone straight up to snap a picture of the earth. In the foreground an iPad displays the picture he snaps:

It is sunrise. The astronaut sees a half-earth. The long shadows stretching west aren't wholly dark, they are lit by the half-earth above.

As the sun climbs towards high noon, earth is a waning crescent.
At noon the earth is at it's dimmest being a very thin crescent or a new-earth. But the moon remains well lit because it's high noon. Except on rare occasions when the sun passes behind the earth.

As the sun sinks towards the horizon, earth is a waxing crescent.

At sunset the waxing crescent has grown to a half-earth. The long shadows stretching east are lightened by the half-earth above.

As the sun sinks deeper behind the horizon, earth is waxing gibbous.

At midnight the astronaut sees a full-earth. This full earth is 34 times brighter than the full moon earthlings see.

From midnight to sunrise, the astronaut sees a waning gibbous earth. At sunrise we're back to where we started.

To The Dark Side

The astronaut hops in his buggy and starts driving east. As he drives closer to the far side, the earth sinks toward the horizon. When the earth is near the horizon it's possible for the sun to be below the horizon when the earth is a dim thin crescent. Even so, the astronaut enjoys strong earthlight for most the night.

When the astronaut drives over into the far side, there's no earthlight. On the far side, it's a deep stygian blackness during the two weeks from sunset to sunrise.

The far side is dark in another sense. Earth is a bright radio source. The far side of the moon is always shadowed from earth's radio noise. Radio astronomers salivate at the thought of a radio telescope under the far side's dark skies.

So you see, the Pink Floyd lyrics make some sense even if you're not under the influence.


Warren Platts said...

Yes, I always liked the phrase "dark side" of the Moon. Now I know why. Thank you!

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