Today’s topic is just for fun.

I am naturally a curious person so I appreciate explanations to things that I have seen but can’t explain. I was reminded about this topic recently when I was driving home from Denver and saw the moon on the horizon. The moon looked gigantic! Then I recalled that in Optometry school I was taught about this phenomenon.

So what are the possible explanations for this phenomenon? I know that before Optometry school I had come up with a few explanations. Does it have something to do with visual queues when it is on the horizon? Does the transition from the vacuum of space to the atmosphere cause some magnification variations? What are other explanations?

Of the above explanations, the first I listed is closest to the real answer. To prove this you could do a little experiment. Some materials you might need are a magnifying lens (a lens from a +2.00 over the counter reading glasses would work great), a piece of paper, and a millimeter ruler. Next find a time when the moon is at the horizon. Hold your lens up to the moon and hold the paper behind the lens so that the moon, the lens, and the paper are all in line. You should be able to see an image of the moon on your paper.

Next you need to make sure that the image of the moon is in focus so you move the paper back and forth until the moon image is focused. For a +2.00 lens this will be at about 50 cm (20 in). Next, measure the image of the moon on your piece of paper. This same procedure would then be repeated when the moon is somewhere other than at the horizon. I guarantee that you will measure the image of the moon on your paper the same diameter every time (with only a small margin of error). This means that the image size when it is on your eye will be the same every time.

So why does it look bigger? To better explain this, I will use the picture below to help me:

Moon illusion

The outer circular pattern is the actual path of the moon. This demonstrates the point that I just made. However, the inner path is what our brain perceives as the path of the moon.

So when the moon is on the horizon, our brain (not our eyes) says that it is really far away. However when it is straight up, because there are no other visual queues, the brain seems to think it is closer than it really is. Then your brain plays more tricks on you by saying, “well if the moon is closer it should look bigger, but it doesn’t look bigger!”

Any time the brain encounters these conundrums it has to make up the difference somehow. Often the solution is to change your perception of reality. In this case, to make up the difference, the brain decides that the moon is big on the horizon and small when it is straight up. So now you know a little more about the moon. I hope you had as much fun learning about this as I did.

If you have questions about any other optical illusions or other visual issues, please feel free to post comments. And as always, you can give us a call and schedule an appointment (307) 265-7008