We have record of people noticing that the Moon looks larger at the horizon than directly overhead dating as far back as 4 BC, but I have a feeling that people have noticed it for as long as people have noticed the moon – which is really hard to miss. Ever since then, we have told and re-told tales of possible explanations of the phenomena regardless of the truth behind the tale. One outrageous idea suggests that the Moon might actually be moving closer to or farther away from the Earth to make it appear larger or smaller in the sky. Another possible explanation suggests that the round shape of our atmosphere might act like a magnifying glass at the horizon, making the moon look bigger, which has since been proven to be untrue. In fact, scientists haven’t actually agreed upon one single explanation amongst themselves. As you might guess, the answer is a lot more complicated that anyone previously thought.
One thing scientists know for sure about the Moon illusion is that it’s definitely an illusion that manifests in our minds because the angular size of the moon is the same no matter where its position is in the sky. This can be made obvious by the use of a theodolite or by taking images of the moon at various positions in the sky. A much simpler yet less precise way to estimate the size of the moon in the sky is to hold your thumb out at arm’s length beside the moon and compare it to your thumb nail while it is at various positions in the sky.
A popular explanation has to do with how our eyes focus on close and distant objects. When we look at the moon directly overhead it lacks the visual cues to tell our brain that it is really far away so our eyes focus on it as if it were closer. Oddly enough, when our brain focuses on the horizon moon and we perceive it as being farther away it actually makes it appear closer. This seems backwards, but it can be demonstrated if you imagine you are looking down a street and there’s a transfer truck 100 yards away and a tiny sports car 10 feet away in the same visual field. The truck is still perceived as being larger even though it produces a smaller image onto your eye.
There are also quite a few optical illusions that are referenced when discussing the Moon Illusion, which include the Ponzo Illusion and the Ebbinghaus Illusion.
Example of the Ponzo Illusion
Example of the Ebbinghaus illusion
None of the explanations are exactly solid because the same moon illusion is experienced by airplane pilots and even astronauts on The International Space Station who never have long stretches of landscape surrounding the moon to give a visual comparison. It is also worth mentioning that because of the eccentricity of the Moon’s orbit it does appear about 11% larger during part of the year, but that’s hardly noticeable over that amount of time unless you’re specifically looking.
We may never pin down the exact reason why we perceive the moon as being larger at the horizon, but I am glad I get to witness such a grand illusion as frequently as this one!