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MCAT Verbal Passage 1 Question 1

Why does the Moon appear bigger at the horizon than up in the sky? Astronomers call this old question the “Moon illusion,” but the same thing is observed with the Sun. And even constellations, as they rise in the sky, appear smaller and smaller. Obviously, none of these objects actually changes sizes or shape, so why do they seem to grow and shrink?

Although there is not total unanimity on the subject, astronomers — for the most part — are satisfied that three explanations answer this question. In descending order of importance, they are:

(1) As Alan MacRobert of Sky & Telescrope magazine states it, “The sky itself appears more distant near the horizon than high overhead.” In his recent article in Astronomy magazine, “Learning the Sky by Degrees,” Jim Loudon explains, “Apparently, we perceive the sky not as half a sphere but as half an oblate [flattened at the poles] spheroid — in other words, the sky overhead seems closer to the observer than the horizon. A celestial object that is perceived as ‘projected’ onto this distorted sky bowl seems bigger at the horizon.” Why? Because the object appears to occupy just as much space at the seemingly faraway horizon as it does in the supposedly closer sky.

(2) When reference points are available in the foreground, distant objects appear bigger. If you see the Moon rising through the trees, the Moon will appear immense, because your brain is unconsciously comparing the size of the object in the foreground (the tree limbs) with the Moon in the background. when you see the Moon up in the skin, it is set against tiny starts in the background

Artists often play with distorting perception by moving peripheral objects closer to the foreground. Peter Boyce, of the American Astronomical Society, adds that reference points tend to distort perception most when they are close to us and when the size of the reference points is well-known to the observer. We know how large a tree limb is, but our mind plays tricks on us when we try to determine the size of heavenly objects. Loudon states that eleven full moons would fit between the pointer stars of the Big Dipper, a fact we could never determine with our naked eyes alone.

(3) The Moon illusion may be partially explained by the refraction of our atmosphere magnifying the image. But even the astronomers who mentioned the refraction theory indicated that it would explain only some of the distortion.

A few skeptics, no doubt the same folks who insist that the world is flat and that no astronaut has ever really landed on the Moon, believe that the Moon really is bigger at the horizon than when up in the sky. If you want to squelch these skeptics, here are a few counter-arguments that the astronomers suggested:

Take photos of the Moon (or the Sun) at the horizon and up in the sky. The bodies will appear to be the same size.

Cover the Moon with a fingertip. Unless your nails grow at an alarming rate, you should be able to cover the Moon just as easily whether it is high or low.

Best of all, if you want proof of how easy it is to skew your perception of size, bend over and look at the Moon upside-down through your legs. When we are faced with a new vantage point, all reference points and size comparisons are upset, and we realize how much we rely upon experience, rather than our sensory organs, to judge distances and size. We do, however, suggest that this physically challenging and potentially embarrassing scientific procedure be done in wide-open spaces and with the supervision of a parent or guardian. We cannot be held responsible for the physical or emotional well-being of those in search of astronomical truths.

From D. Feldman “Why Do Clocks Run Clockwise?” and Other Imponderables. 1987 by David Feldman.

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QUESTION #1: According to the passage, a comparison of photografts of the Moon (or the Sun) taken when it is high overhead and when it is near the horizon would show that:

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A. any difference in their apparent diameters could be discerned only by very careful measurement

B. The disks are the same size

C. The lunar disk is precisely the same size as the solar disk

D. the Sun overhead is the same size as the Moon at the horizon

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ANSWER

The answer is B

– Taking photographs of the Moon at moonrise and again when it is directly overhead is the first suggestion the author gives for “squelching skeptics” who doubt the lunar disks in both photographs would be of the same size (lines 55-57).

– Incidentally, the disks of the Sun and Moon as we see them here from Earth actually are very close to the same apparent size too — about half a degree of arc in diameter — which you can appreciate best with the naked eye during a total solar eclipse; so some of the other answer choices offered for this question are technically not far off the mark.

– But you were asked about what the passage says, so stick with B.

– The correct choice is B.

SOURCES

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