The dark side of the full moon

The Moon’s true color and brightness is obvious as it passes in front of Earth, with North America just above it. (Taken by NASA’s climate Explorer spacecraft)

When the Full Moon arrives next Saturday night, the 27th, let’s finally learn its brightness. For, in the mass media the past 20 years or so, all sorts of make-believe things have been presented about the Full Moon.

Starting with its name. Newly coined labels like blood moon, blue moon and super moon have become popular, along with the false assertion that there are actual official names for each month’s Full Moon, like “Wolf Moon,” or “Strawberry Moon.” In truth, each Native American tribe had their own names for each Full Moon, so that if the Algonquin called next weekend’s Full Moon “The Snow Moon,” the Nez Perce called it “The Budding Time Moon.” There’s no accepted name except for autumn’s Harvest and Hunters moons.

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With all such ambiguity, let’s explore aspects that ought to be straightforward — its color and brightness. 

The Full Moon certainly seems dazzlingly white. Expressed by the percent of sunlight an object reflects (its albedo) the Moon might seem at least 50% reflective. Textbooks, however, tell us that the average lunar albedo is 11. 

This is amazing. Dark forest foliage has an albedo of 15. Coal is 5. So the Moon’s surface reflectivity lies somewhere between coal and dark green leaves, and matches that of an asphalt driveway. Super murky. So why does it seem so white and bright?

That’s because our eyes reset brightness levels according to the surroundings. The Full Moon sits against a black background sky, so our brains paint it white. But if we could somehow see it alongside earthly objects? Amazingly, NASA’s Climate Explorer spacecraft recently accomplished exactly that. Orbiting a million miles from us at a point where Earth’s gravity balances with the Sun’s, it always views a “Full Earth.” Twice a year it aligns with the Moon’s tilted orbital plane so that it catches the Moon passing in front of us. Then its images confirm that the Moon has ¼ our diameter. But even better, it then also shows that the Moon has about ¼ our albedo, our brightness, revealing the Moon is a very dark object!  

It’s then also peering at the lunar hemisphere facing away from Earth, the one that strangely has very few Maria, the dark blotches that dominate our own naked eye lunar view. So it always sees the brightest lunar hemisphere. And yet, look at it! Here at last, after two-million years of humans gazing moonward, we at last view the Moon against an earthly setting instead of a black sky. And finally learn firsthand that the Moon is a miserably dim body. 

This picture barely made a splash when it was released by NASA a few years ago. But let’s now let it sink in. The true Moon. The actual dim, dark, desolate Moon.