The
Moon's Illusion

There is something rather strange about a full moon -- an effect that nearly everyone has noticed because it is so apparent, and yet it is nothing more than an illusion. And even more curious, the reasons for it are unknown.

Just after they have risen, full moons look bigger than at any other time they are in the sky. Think about it. You've seen the just-risen full moon. There it sits, barely above the horizon, glowing big and bright, fat and round. But after it gets much higher overhead, it doesn't look quite as big as it did just a few hours earlier. Yet, common sense will tell you the moon is the same size regardless of where it is in the sky, and so it should appear to be the same size. But it doesn't. Ask anyone whether the moon looks bigger on the horizon than it does overhead and you'll almost certainly get a "yes" response. But in reality, there is no difference in size. Why does the moon fool us this way?

This celestial illusion has been noticed for centuries. The sun has the same effect. It appears fat and distorted near the horizon. But because of its tremendous brilliance, it is impossible to observe it overhead without special equipment. None is needed for the moon, of course, thus the effect is more noticeable. Let's look at some possible explanations.

Could atmospheric effects cause the illusion? The atmosphere acts similar to a lens which compresses the sun's (and the moon's) vertical dimensions, accounting for the oval shape. However, this would actually make the dimensions of the disc less, and should make it appear smaller, not larger. Besides, looking straight across to the horizon means you're looking at the moon across the radius of the earth, and you've added some 4,000 miles to the distance, further reducing the apparent size of the disc you see.

In 1959, tests were conducted at a Wisconsin university. A disc some two feet in diameter was suspended 85 feet above the ground, and an identical disk placed 85 feet away horizontally. Subjects were asked to stand at a point equally distant from both discs without being told they were the same size. When questioned, every one of the participants said they thought the disc on the horizon looked bigger. Again, the test was performed -- this time in a darkened room. The results were the same.

You can conduct your own experiments. Next full moon, observe it carefully as it rises, but look at it through a hollow tube which blocks out as much of the surrounding horizon scenery as possible. If you take away the point of reference the horizon provides, the full moon looks much smaller. Another method is to lie down and look at the moon from a flat position. The moon seems smaller from a lying position than a standing one. Or, try this: stand with your back to the moon, bend over and observe the disc between your legs. Again, the moon appears smaller than it does from a normal standing position. As you ponder the reasons why the illusion has such a perplexing aura about it, you'll be experiencing the same riddle that faced the great astronomer Ptolemy, who recognized the same effect in 150 A.D.

However, if you remain unconvinced, here is another experiment that will prove to you the full moon on the horizon is the same exact size as the full moon overhead. An aspirin tablet held lightly between the thumb and index finger at arm's length is nearly the same apparent size as the full moon. It will nicely cover the disc of the full moon regardless of whether it's grinning at you just over the horizon or riding high overhead. Give it a try and see for yourself.