Have you ever thought your eyes are playing tricks on you? Depth perception cues account for many of these instances.
A few years ago I was driving home in the evening just as the full moon was rising. It was rather low on the horizon and I thought about how large it looked, larger than usual, actually. A few hours after I got home I remembered the enormous full moon and excitably brought my dad outside to see how huge it was. I was disappointed to find that it had shrunk from its size earlier in the evening. Did the moon actually shrink?
Of course, the moon didn’t actually change size. I was experiencing an example of size constancy and apparent-distance hypothesis (Ross). There is a similar effect when you try to take a picture of the moon; you end up being extremely disheartened to find that you just end up with a black photo with a small white dot.
This is because you don’t have anything else in the picture to relate the size of the moon to. The reason objects near the horizon seem bigger is similar. When the moon is near the horizon, you have other depth cues to relate the moon to. When it is in the middle of the night sky, there is nothing surrounding to compare (Ross).
To further exam depth cues, let’s look at George Seurat’s famous painting Sunday Afternoon on the Island of La Grande Jatte. There is a park full of people in the foreground and the background and we perceive all the people to be the same size. There are no abnormally large or small people. If you look more closely at this painting, the people are all different sizes, depending on their apparent distance. The people in the foreground are larger and the people in the background are smaller. If all the people were brought to the foreground, we would expect them to be the same size. Our brain calculates the distances and adjusts the size of the apparent objects to fit what we expect them to be based on depth cues (Robinson).
Coming back to the moon example, we can further examine the apparent-distance hypothesis. When the moon is in the middle of the sky, we calculate it to be closer and see it as a certain size. When the moon is on the horizon, that is our sort of background and because we perceive this as farther away, we calculate it to be larger (Robinson). If two objects of the same size are put in a plane, one in the foreground and one in the background, we will perceive the one in the background to be larger.
Next time there’s a full moon; don’t let your perception fool you. Believe it or not, the moon actually stays the same size.
References:
Robinson, Richard. Why the Toast Always Lands Butter Side Down. New York: Barnes & Noble, 2005.
Ross, Helen. (2004). “Moon Illusion.” Retrieved from http://www.answers.com /topic/moon-illusion
