The question in How Come Aug. 11 about the eerie "moon illusion" prompted gridlock at the Horizon mailbox. Many readers wrote to offer inventive alternative explanations of the famous psychological delusion, in which the moon near the horizon appears at least 50 to 75 percent larger than it does at zenith.
Hermann O. Pfrengle of Herndon suggested that "the more obvious explanation is that near the horizon the moon is seen through a wider mass of atmosphere than when it's at higher angles" and the atmosphere, laden with vapor and dust, "can enlarge the images of objects beyond it."
Physicists who have studied the effect, however, note that such atmospheric blurring might alter the moon's apparent size by a tiny amount but could not possibly account for the very large difference in perceived size. You can see this yourself by examining time-lapse pictures of the moon climbing in the sky -- the size remains the same.
In the same vein, Tim Hubbard of South Riding, Va., wrote that "in a geography class in college," he was taught that the moon illusion comes from the same effect that "causes the sun to change from an orange/yellow when directly overhead, to a red color when on the horizon.
"When it is on the horizon, one is looking through a greater [amount] of atmosphere containing dust particles and water molecules. . . . Is there any truth in this, or should I ask for a refund?
It is absolutely true that the quantity of air and airborne particles through which light passes will change its color, scattering the smaller wavelengths (violet through green) much more than the longer orange and red wavelengths, which is indeed why sunsets look ruddy. But that has nothing to do with the moon illusion.
Another reader, Kaitlyn Flynn of Manassas, voiced a similar theory that the illusion arises because "when the moon is near the horizon, its light is passing through the atmosphere at a shallower angle, giving it more air to pass through, which allows for greater refraction."
Interesting idea. There is a slight refraction effect as moonlight bends when it enters the atmosphere. But actually, that makes the moon look a bit smaller near the horizon.
Tom and Tess Madden of Bethesda offered an intriguing suggestion: "Here is what we believe to be an accurate description of what happens, which we quote from the 1990 edition of Foundations of Astronomy by Michael A. Seeds: `The moon orbits around the Earth at an average distance of 384,401 kilometers [238,329 miles]. Its orbit is slightly elliptical [emphasis added] so its distance can vary by about 6 percent.' "
For sure. At apogee, its greatest distance from Earth, the moon is 251,978 miles away. At perigee, the closest point, it's 225,755 miles away. That's an impressive difference.
However, it could not have any influence on the moon illusion, which occurs when the observer looks at the moon on the horizon and later directly overhead on the same night. During the course of several hours, the moon's distance from Earth could change by no more than a few hundred miles. That's nowhere near enough to produce the illusion.
Finally, Robert J. Robertory of Fairfax Station wrote, "You state, in speaking of the moon, that `the distance is the same when the moon is at zenith or at the horizon.' This is not correct. The distance from the moon to the center of the Earth remains the same (with an insignificant -- for this purpose -- change due to its elliptical orbit). [See above.]
"Since the observer is on the surface of the Earth, when the moon is at zenith, the observer will be closer to the moon by a distance equal to the semidiameter of the Earth if the moon is over the observer's latitude as well as longitude. This distance will be slightly less than the semidiameter if the moon is over a different latitude. . . ."
That's a fine distinction but, alas, is negligible for our purposes. The moon orbits Earth at an average distance of about 240,000 miles. Even the full Earth radius of about 4,000 miles is only 1.6 percent of that -- an amount that would have an imperceptible effect on a human observer, especially at our latitude.