The Full Moon: Strangely Alien

If the weight of a mouse represents the Full Moon’s brightness, then the brilliance of sunlight would equal the weight of 18 full-grown elephants.

The next Full Moon is Saturday night, December 18. It’ll be the highest-up Full Moon of the year, and will attain that overhead position at midnight.  Very cool, but why tell you all about it more than a month early? It’s sort of a lame excuse, but we mistakenly ran the illustration for it last week, with no caption, guaranteed to produce bewilderment in any sane person. So clip this out and save this article until then — or indeed for any Full Moon. Because you really should realize why they’re so special.

First know this: the Full Moon and the Sun always hover on opposite sides of the sky. So when one is setting, the other is rising. Remembering this will forever let you plan picture-taking, since the Moon always looks more dramatic and large when it’s very low. And it’s low just as evening twilight is underway on the evening of each Full Moon, throwing warm light on foreground earthly objects like low mountains.

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What’s possibly its strangest aspect is that the Moon is the only nightly object known to everyone. Everyone recognizes the Moon but relatively few can identify a single other celestial object at night. (Of course, anyone can say, “That’s a star,” but few can tell you which star.) This isn’t true in other areas of science or nature. People who can name one type of cloud can probably identify a few others too. If you can point out a cardinal, you can likely also identify robins, pigeons, and crows. Only in the night sky does nearly everyone know exactly one object but no further examples.

But despite this widespread familiarity, people are genuinely astounded to learn the Full Moon’s basic realities.

Its apparent size. Okay, how large is the Full Moon? Most folks aren’t familiar with degrees and angles, so phrase it this way: “How many Full Moons would you need to pile one atop the other to stretch from the horizon to a point directly overhead?” Ask your family. Most will guess between 20 and 50 Moons.

You’ll laugh when you survey a bunch of people because you’ll keep hearing that same 20-to-50 range. The reality? You’d need 180 stacked Full Moons to fill that gap. The Moon is far, far smaller than we remember it as being.

Its brightness. Here’s the next misconception: Ask, “How much dimmer is a scene lit by the Full Moon than a sunlit scene?” Their answer will now range from 100 to 1,000 times, meaning, people think sunlight is as much as a thousand times brighter than the Full Moon. The real answer? Sunlight is 450,000 times brighter than the Full Moon.

Its shininess. The Moon looks dazzlingly white. Expressed as a percentage of the sunlight it reflects (its albedo) the Moon might seem to rate at least a 50 — as in, 50 percent reflective. But the average lunar albedo is 11. This is amazing and noteworthy. Dark forest foliage has an albedo of 15. Charcoal is 5. So, the Moon’s surface reflectivity lies somewhere between coal and dark green leaves — super murky. So why does it seem so white and bright?

It’s because the mammalian eye continually resets brightness levels according to the surroundings. The Full Moon hovers against a black background sky, so our brains paint it white. But if we could somehow see it against an earthly background, it would be a very dark object. Indeed, if some crazed developer paved the entire Moon with asphalt, and turned it into one giant mall parking lot, it would continue to appear just the way it looks now!

At the opposite end of the scale, a 5 p.m. glance low to the southwest instantly displays the shiniest object in the solar system. That’s Venus, soon to disappear. But before it does, consider that its own albedo is a whopping 75% — not too inferior to that of a mirror.

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