Moon Illusion
A few nights from now, barring clouds, an astonishingly large, yellowish sphere will loom low in the eastern sky. Children will point. Romantics will swoon. Photographers will run for their cameras.
It’s the annual harvest moon – so big and ever-present that farmers once used its light for gathering the crops. Even Mars, making its closest pass to Earth in 60,000 years, will have to take a back seat for a few nights.
The harvest moon is the full moon that occurs closest to the autumnal equinox (Sept. 23). It begins this year on Wednesday and should provide a parade of glorious moonlit nights. This is the beach-ball-size moon you see on Halloween cards and in children’s books, usually framing a black cat or a witch on a broomstick.
The harvest moon is not really bigger than any other full moon, but because of a quirk in its rising times, it frequents the early evening hours, hanging out near the horizon where every full moon looks strangely large. Why? No one is sure. But, when the moon hits your eye like a big pizza pie, that’s an illusion.
Unlike the sun – reliable in appearance, predictable in its movements – the moon is ever-changing and capricious. It goes through more phases than a teenager and keeps ridiculously irregular hours. It is just as likely to rise in the wee hours of the morning as after sunset and can often be seen wandering ghost-like in the blue sky of day.
It’s evasive. You might think, for example, that a huge spherical object revolving around us every month, spinning all the while, would have nothing left to hide. Yet the moon, through a series of exquisitely timed movements, manages to continually conceal its back side from earthbound observers.
And paradoxical. Its feeble moonbeams couldn’t warm a drop of water, yet it’s the most dynamic force on Earth, yanking our immense oceans to-and-fro in daily tidal rhythms. It’s a cold, lifeless body 240,000 miles away, yet its 28-day orbital period sets the clock for women’s reproductive cycles.
The harvest moon provides one of the best opportunities to observe the large moon illusion because its moonrises are spaced more closely together than the norm. On average, the moon rises 50 minutes later every day (a progression mimicked by the tides). The first full moon rises in the East, just after sunset. A week later, with half its disk in shadow, it’s coming up around midnight. But with the harvest moon, the period between moonrises is cut roughly in half (even less farther north), so the moon is up in prime time for several nights running.
So, what makes it appear so big – up to twice its normal size?
Well, first of all, it’s nothing the moon is doing. The moon has a circular orbit, so the distance between it and Earth changes very little.
Another possibility, first put forth by the second century astronomer Ptolemy, is that it’s an optical effect of the physical world – the image is magnified by some lens-like property of the atmosphere.
Scientists say that doesn’t seem to be the case, either, although the atmosphere does affect the moon’s color. No, the image of the moon that comes to our eyes is exactly the same size whether it’s at the horizon or straight overhead.
You can test this yourself by looking at the moon through a rolled up tube of paper, or by holding a dime at arm’s length and covering the disk. In both cases, the moon will prove to be the same size no matter where it is in the sky. This is why people who try to photograph the big moon are so often disappointed. The prints tend to show an ordinary looking orb where there was supposed to be a gargantuan one. (Experienced photographers learn to simulate the effect with a telephoto lens.)
So, whatever is happening happens inside our heads.
Is it any wonder that the moon has a reputation for undermining human reason?
An 11th century Egyptian scholar, Ibn Alhazan, offered the most common-sensical explanation – that seeing the moon in relation to familiar objects on the ground, such as trees or buildings, dramatizes its size, like a blimp suddenly dropping down in your back yard.
Anyone who has seen the moon coming up behind the New York City skyline can vouch for this theory.
And there’s support for it in a rather bizarre experiment that might be called “mooning the moon.” Turn your back to the moon, bend over, and look at it through your legs. (If nothing else, you’ll be lending support to the reputed link between the moon and lunacy.) The big-moon effect disappears.
Is this because an upside-down world no longer provides the familiar context against which to measure the moon?
That sounds reasonable, but it can’t be the whole story, because the enlargement is just as pronounced over the ocean where there are no objects to compare it to. And people looking out the windows of airplanes also observe it.
The most commonly accepted theory has to do with the way our brain evaluates the size of distant objects. It starts with the notion that we unconsciously perceive the sky as a dome —like the inside of a planetarium —with the celestial objects moving about on its surface. But this dome is not conceived as perfectly round – the distance to the horizon is seen as greater than the distance to the apex of the dome.
There is some truth to this, and it accounts for why the moon often looks orange or yellow when it’s low in the sky. When we look at the moon at the horizon, we are seeing it through more atmosphere than when we look at it straight overhead. The atmosphere extends out from the Earth about 621 miles. When we look across the earth’s surface at an object out in space, however, we look through the low-lying atmosphere between the horizon and us, plus the 621-mile thickness straight out into space.
This additional atmosphere, laden with dust and other particles, scatters the moon’s light -and shifts it toward the red-yellow end of the spectrum, accounting for the harvest moon’s characteristic hue.
So far, so good, except the mind exaggerates this extra horizontal distance, which, relative to the total distance to the moon, is actually insignificant. The mind, convinced otherwise, faces an internal conflict that might go something like this:
Perceptual Control: “Why doesn’t the faraway moon look smaller than the closer, overhead one?”
Command Central: “It must be bigger than we thought.”
Perceptual Control: “There are two moons?”
Command Central: “Never mind. Just make it look bigger.”
Perceptual Control: “But … ”
Command Central: “Do as you’re told.”
Perceptual Control: “OK, you’re the boss.”
So that’s what we see. A moon of our own making. Well, except for a small percentage of people whose brains refuse to go along with this self-deception. Good for them. But they’re missing quite a sight.
