One of the morenoted optical illusions in the annals of visual deception appeared in the night sky this week when a giant full moon appeared on the horizon.
The giant ball of yellow-white light made the Moon appear to linger for longer, and Earth's satellite seemed larger at that point in its journey across the sky than at any other position.
In fact, the size of the Moon in the sky does not vary and its apparent enlargement on the horizon is purely a trick of the eye. It was first written about by the ancient Chinese and Greeks – although scientists are still arguing over its cause.
The illusion was especially visible on Wednesday night because this month's full moon coincided with the summer solstice, while clear skies gave spectacular views as the moon rose slowly above the horizon.
When the Moon is full, it and the Sun are on opposite sides of the sky. During summer, when the Sun rises high in our sky, the full moons are correspondingly low – allowing them to linger longer over the horizon.
The astronomer Sir Patrick Moore said that there was no doubt that when a full moon was low on the horizon it invariably looked bigger than when it was high up in the sky, but that this was purely an illusion.
"The effect is visible at every full moon but was particularly good this time because the full moon was as low in the sky as it could ever be and appeared to hover above the horizon," Sir Patrick said.
One way of showing that the Moon does not really vary in size is to hold up a small coin to the sky to see how far away it can be held before it blocks out the Moon – it should do it at the same distance no matter where the Moon is positioned.
"It has been known and commented on for many hundreds of years. An explanation was given by the last and greatest astronomer of ancient times, Ptolemy, who said the illusion was due to the fact that we were seeing the Moon across filled space and could compare it with objects such as trees and houses," Sir Patrick said.
The conventional explanation for the illusion today is that it is a combination of two psychological effects.
The first is the Ponzo illusion, named after Mario Ponzo in 1913, who drew two identical bars across converging lines such as railway tracks. Both bars are the same size but the nearer looks smaller because the eye and the brain judge the farther bar to be bigger in proportion to its surroundings. This could explain why we perceive the Moon to be larger when it lies against a background of familiar objects such as trees and buildings.
However, critics of this idea point out that airline pilots flying at high altitudes sometimes experience the Moon illusion even without seeing any recognisable features on the horizon. And, curiously, the illusion disappears for many people when they bend down and watch a full moon through their legs.
The second explanation is that the brain does not see the sky as the "ceiling" of a true hemisphere, but rather as a flattened dome. In other words, objects that are overhead, such as flying birds, are perceived to be nearer than objects on the horizon – which is why we imagine objects on the horizon to be bigger than objects overhead.
But this explanation does not satisfy some scientists either. In a theory similar in some ways to the Ponzo illusion, Professor Don McCready of the University of Wisconsin believes the true explanation rests in the fact that our brains tend to make objects smaller when they appear closer to us based on distance cues. When the Moon is on the horizon, the surrounding buildings and trees give us clues that it is very far away which causes the brain to perceive it to be larger than it would normally appear, Professor McCready said.Reuse content