Moon Illusion: Why Does the Full Moon Look Bigger on the Horizon?
Every once in a while, images of a massive Moon close to the horizon appear in the media. Does the Moon increase in size on some nights?
An Optical Illusion
The simple answer to this question is, no. While the Moon does come closer to our planet during its 29.5-day orbit around the Earth, and while it does sometimes look bigger than usual to a casual observer on Earth, its size does not actually increase. The apparent change in our only natural satellite's size when it is near the horizon is an optical illusion. Scientists call it the Moon illusion.
Confirm on Your Own
You can easily confirm that there is no change in the Moon's size when it is on the horizon versus when it is high up in the sky by doing a very simple experiment. Take your camera and take a picture of the Moon when it is at the horizon. Wait a few hours. Now, using the same settings, take another picture of the Moon. Compare the sizes of the Moon in the two pictures. You'll find that they are the same.
Another way to test this is to take a sheet of paper and roll it such that the edges of the roll match with the edges of the Full Moon at the horizon. Tape the roll in place. Wait for a few hours and then look at the Full Moon high up in the sky through the roll. You'll find that the edges of the roll perfectly fit the edges of the Moon once again.
Observed Since Ancient Times
While people have been aware of the illusion since ancient times, an explanation of why it occurs has been difficult to pin down.
As early as the 4th century B.C.E, the Greek philosopher Aristotle noticed that the Moon looked bigger when it was closer to the horizon than when it was further up in the sky. The popular explanation for the optical trick at that time was that the Earth’s atmosphere magnified the Moon.
Scientists have now debunked that explanation—the Earth’s atmosphere does have an effect on the color of the Moon, but it does nothing to perceptibly change its size.
It's in Our Heads
So what causes the illusion? Scientists are unsure. One popular explanation suggests that how we perceive the size of the Moon is all in our head. In other words, the Moon illusion has psychological roots.
Humans tend to mentally exaggerate the size of the Moon with respect to the surrounding objects when it is on the horizon. This is because, unlike other everyday objects in the sky—airplanes and birds—humans have no context to determine the size of celestial objects. According to one psychological explanation of the Moon Illusion, this can force people to believe that the Moon is bigger when compared to the objects at the horizon like trees and buildings. This is similar to the Ebbinghaus illusion, which shows that when a circle is surrounded by larger circles, it looks smaller than when it is surrounded by smaller circles.
The Sky Illusion
Another explanation lies in how humans tend to perceive the sky. Known as the apparent distance theory or the sky illusion, the explanation was made popular by Arab scientist Ibn al-Haytham. He suggested that man-made and natural objects between an observer and the horizon create an illusion, where the observer inflates the distance between him and the horizon as compared to the distance between him and those directly above him. This leads to the observer believing that the horizon Moon is further away and bigger than the Moon at or close to the zenith.
Another version of the sky illusion is the flat sky theory or the apparent sky dome theory. According to this theory, the human brain perceives the sky above us not as a dome, but as a flattened dome, much like an inverted bowl. When the Moon is projected on this mental model of the flat sky, the brain sees the Moon at the horizon as bigger than the Moon on the top of the dome. This is perhaps why pilots see a larger than usual Moon despite not having any intervening objects between them and the Moon.
Scientific experiments have however proven that in general, people tend to presume that the Moon is bigger and closer to the Earth when on the horizon.
The Ponzo Illusion
Similar to the sky illusion, the Ponzo illusion, named after Italian psychologist Mario Ponzo, suggests that when two identically sized lines are drawn across a pair of converging horizontal lines, the line at the top is thought to be bigger than the line at the bottom. This is because to the human eye the top line seems to span a greater distance between the two converging lines.
Some experts suggest that objects between the observer and the horizon Moon act as the converging line, tricking the mind into thinking that the Moon is bigger than it is.
Actually Smaller at the Horizon
What makes the Moon illusion particularly confounding is that in reality, a rising Full Moon near the horizon is about 1.5% smaller as perceived by the human retina than an overhead Moon. This is because around the time of moonrise the Moon is about 6400 kilometers (about 4000 miles) farther away from an observer's location than when it is high in the sky.
Sometimes when the Moon is right near the horizon, it can look flatter - as if one of its edges have been squished. This is a mirage, and unlike illusions that have psychological basis, can be explained by the laws of physics. Refraction to be precise. As moonlight passes through the layers of atmosphere, it gets bent. At the horizon, moonlight from the lower edge of the Moon passes through more of the atmosphere than from the upper edge, making its lower edge seem flatter and more distorted than its upper edge.