The Moon Illusion

For a long time, I was convinced that the Moon Illusion wasn’t true. No matter how many scientists discussed what it was and theories about why it might work, I just didn’t believe it. Maybe you've heard about this trick our eyes play on us... when you look at the moon low on the horizon, it appears to be huge, much larger than when it is seen up higher in the night sky. However, if you really take the time to analyze it (maybe by checking to see if there are any features that are clearer when the moon is nearer the horizon, or by holding up a penny at arm's length for size comparison between low and high moons), you'll find that there really is no difference in the apparent size of the moon, no matter where it is the sky. Sure, while it's on the horizon, the moon might be a little distorted and orangey, thanks to the larger amount of atmosphere you're looking through to see it, but for all intents and purposes, the moon appears no bigger on the horizon than it does when it's directly overhead.

Like I said, I used to be skeptical of this. My eyes told me that the moon had to be bigger when it was low in the sky, just had to. I could look and verify it on any clear night. There was no shaking me from that thought. Now, I have to admit, I realize that I was wrong. And what brought me to this conclusion was a simple thought experiment that taught me not to believe everything my eyes think they see.

Start off by thinking about how you judge the distance of objects, especially objects overhead. The ceiling above you right now, for example. If you're sitting down, the ceiling of an average room would be, say, about four feet over your head. As you look at the ceiling toward the corners of the room, the amount of space you have to look across goes up markedly. If you're at the end of a long hallway, the distance from your eyes to the upper corner of the ceiling at the far end is longer than the actual hallway itself, because you're looking across the space at a diagonal. This holds true even when you’re outside -- the distance of clouds near the horizon is clearly much farther than clouds that are directly over your head. So this is the framework your mind is constantly working in, in terms of perspective: directly overhead is closest, the horizon is very far, and the more you lower your gaze, your distance from objects at the same level goes up exponentially.

Now think about the moon. Your actual distance to the moon when it's on the horizon and when it's directly overhead differs by only about 4,000 miles (the radius of the earth -- sketch it out on a piece of paper and you'll see what I mean). Given that the moon is almost 250,000 miles away, that's not a lot of difference. So while in every other earthly situation, items an equal height off the ground will be hundreds of times farther from your eye depending on whether they're right over you or just above the horizon, when it comes to the moon there’s only a change of less than 2%.

And that's how your eye gets fooled. In this unique instance, you're looking at an object that barely changes distance from when it's overhead to when it's actually coming over the horizon. Your brain processes it in the same way it processes perspective to any other object, and the result is that it thinks the moon on the horizon has got to be much larger than when it's above. That’s the only way your mind can figure that it looks to be about the same size. This conflict between its apparent size and the brain's judgment of how far it must be given that it's close to the horizon is what causes the Moon Illusion.

I've heard many people talk about the Moon Illusion, how it's perplexed scientists for centuries, and I've also heard it explained in confusing ways like "because it's near the horizon, your mind processes it like a background object instead of a foreground object", but once I realized my own personal explanation for it, all my confusion just drained away. It made me realize what power there is in scientific acceptance, that beautiful moment when you realize that what you were so sure of five minutes ago is dead wrong, and that it’s okay.

I think a lot of what causes the problems the world has today is our inability to realize when we've been wrong, and to shift our beliefs accordingly. There are so many people holding onto antiquated ideas and dogmas, simply because it's what they're familiar with. I realize that it's scary to give up a belief system you've had for so long, even for so long that it's one you inherited from older generations you love and admire. But keep in mind that many truths we now know, at one time was thought to be either wrong or simply unknowable. Hold fast to what you believe, but not so tightly that you can't let go when it's time to.