The reason for our perceiving the sky this way isn’t well-known. An Arab researcher named Al-Hazan proposed in the eleventh century that this is due to our experience with flat terrain. When we look straight down, the ground is nearest to us, and as we raise our view the ground gets farther away. We interpret the sky the same way. This time as we look straight up, the sky appears closest to us, and as we then lower our gaze the sky appears farther away. Although this explanation is nearly 1,000 years old, it may indeed be the correct one.
But no matter what the cause, the perception persists. The sky looks flat. As Al-Hazan pointed out, this means that the sky looks farther away at the horizon than it does overhead.
Now we can put the pieces together. The Moon, of course, is physically the same size on the horizon as it is overhead. The shape of the sky makes the brain perceive the Moon as being farther away on the horizon than when it’s overhead. Finally, the Ponzo Illusion shows us that when you have two objects that are the same physical size but at different distances, the brain interprets the more distant object as being bigger. Therefore, when the Moon is on the horizon, the brain interprets it as being bigger. The effect is very strong and has the same magnitude as the Ponzo illusion, so it seems safe to conclude that this is indeed the cause of the Moon Illusion.
This explanation was recently bolstered by a clever experiment performed by Long Island University psychologist Lloyd Kaufman and his physicist son, James, of IBM’s Almaden Research Center. They used a device that allowed subjects to judge their perceived distance from the Moon. The apparatus projected two images of the Moon onto the sky. One image was fixed like the real Moon, and the other was adjustable in size. The subjects were asked to change the apparent size of the adjustable image until it looked like it was halfway between them and the fixed image of the Moon. Without exception, every person placed the halfway point of the horizon Moon much farther away than the halfway point of the elevated Moon, an average of four times farther away. This means they perceived the horizon as four times farther away than the zenith, supporting the modified Ponzo Illusion as the source of the Moon Illusion.
However, some people argue with this conclusion. For example, when you ask someone, “Which do you think is closer, the big horizon Moon or the smaller zenith Moon?” they will say the horizon Moon looks closer. That appears to directly contradict the Ponzo Illusion explanation, which says that the brain interprets the bigger object as
However, this isn’t quite right. The Ponzo Illusion is that the farther-away object is bigger, not that the bigger object is farther away. See the difference? In the Ponzo Illusion the brain first unconsciously establishes distance and
In my opinion, the Ponzo Illusion coupled with size constancy and the shape of the sky is an adequate solution to the millenniaold Moon Illusion mystery. The real question may be why we perceive all these different steps the way we do. However, I am not a psychologist, just a curious astronomer. I’ll note that as an astronomer, I am not fully qualified to judge competing psychological theories except on their predictions. It’s quite possible that eventually a better theory may turn up, or that a fatal flaw in the Ponzo Illusion theory may arise. Hopefully, if that happens, the psychologists can explain it to astronomers so we can get our stories straight.
As an aside, I have often wondered if astronauts see this effect in space. One way or another, it might provide interesting clues about the root of the illusion. I asked astronaut Ron Parise if he has ever noticed it. Unfortunately, he told me, the Space Shuttle’s windows are far too small to get an overview of the sky. Perhaps one day I’ll see if NASA is willing to try this as an experiment when an astronaut undergoes a spacewalk. He or she could compare the size of the Moon when it’s near the Earth’s limb, its apparent outer edge, to how it appears when it is far from the Earth and see if the size appears to change. Interestingly, the experiment could happen much faster up there than here on Earth: the Shuttle’s 90-minute orbit means they only have to wait 22 minutes or so between moonrise and when it’s highest off the limb!
Having said all this, I’ll ask you a final question: if you were to look at the full Moon and hold up a dime next to it, how far away would you have to hold the dime to get it equal in size to the full Moon?
The answer may surprise you: over 2 meters (7 feet) away! Unless you are extremely long-limbed, chances are you can’t hold a dime this far away with your hand. Most people think the image of the Moon on the sky is big, but in reality it’s pretty small. The Moon is about half a degree across, meaning that 180 of them would fit side by side from the horizon to the zenith (a distance of 90 degrees).
My point here is that often our perceptions conflict with reality. Usually reality knows what it is doing and it’s we, ourselves, who are wrong. In a sense, that’s not just the point of this chapter but indeed this whole book. Maybe we should always keep that thought in mind.
Part III
Skies at Night Are Big and Bright
If we dare journey beyond the Moon looking for bad astronomy, we’ll find a universe filled with weird things waiting to be misinterpreted.
Meteors are a major source of bad astronomy. When two eighteenth-century Yale scientists proposed that meteors were coming from outer space, one wag responded, “I would more easily believe that two Yankee professors would lie than that stones would fall from heaven.” That wag was Thomas Jefferson. Thankfully, he stuck to other things like founding the University of Virginia (my alma mater) and running the country, and steered clear of astronomy.
If you go outside on a cloudless night, you might see a meteor or two if you’re lucky. If you are not too close to a city and its accompanying light pollution, you’ll see hundreds or even thousands of stars. Like meteors, that starlight has come a long way; even the closest known star is a solid 40 trillion kilometers away. And like meteors, those stellar photons end up as so much fodder for our human misunderstanding of the cosmos. Stars have color, they twinkle, they come in different brightnesses, and all of these characteristics are subject to clumsy misidentification.
Bad astronomy can often force the doomsayers out into the open, too. This happened in the years, months, and days leading up to the “Great” Planetary Alignment of May 2000. Last I checked, the world had not ended. Cries of doom always seem to pop up at solar eclipses as well. Long heralded as omens of the gods’ ill favor, eclipses are actually one of the most beautiful sights the sky provides.
Finally, in this section we’ll travel back in time and space to where it all began, the Big Bang. Something about contemplating the beginning of everything twists our already tangled minds, and descriptions of the Big Bang usually confuse the issue more than unravel it. The irony of the Big Bang, I suppose, is that it is even odder than our oddest theories could possibly suppose.
9.
Twinkle, Twinkle, Little Star: Why Stars Appear to Twinkle
“Twinkle, twinkle, little star, how I wonder what you are.”
“Twinkle twinkle, little planet, can’t observe so better can it.”
