On March 19th 2011 the moon was at its closest point to Earth in 18 years, appearing 14 percent larger and 30 percent brighter in the sky, leading many to dub it the “Supermoon”. This was due to the orbit of the moon, which is not exactly circular. Instead, it goes through motions of closest and furthest approach and, on Saturday evening, it reached its closest point (221,565 miles / 356,575 km) from Earth at almost the same time that it was a full moon, enhancing the ‘Supermoon’ effect. However, this event does not entirely account for how large the moon appeared. For that, we must look into ‘The Moon Illusion.’
Now, this is trickier than it sounds. We’ve all seen how a rising or setting moon on the horizon appears larger than an overhead moon, and yet scientists and psychologists still can’t agree on or understand why it occurs – even NASA can’t fathom it.
There are two main theories behind ‘The Moon Illusion’ that contributed to the ‘Supermoon’. We know the size of the moon doesn’t actually change so we can safely assume that it’s a trick of the mind. One idea suggests the viewer instinctively attempts to judge the distance to the rising moon (it’s hard to comprehend hundreds of thousands of miles) based on visual objects, such as trees and houses in the distance. These objects seem near the moon, giving a distorted point of reference, making it appear bigger. However, this theory can be called into question as pilots have also seen the illusion despite no point of reference against the ground.
The second theory has to do with the fact that we tend to think of the sky as a flattened dome, rather than the hemisphere it is, and therefore perceive things overhead (birds and planes) as much lower, or nearer than the things we see on the horizon. And so although the moon may well be the same size whether it’s above your head or off on the horizon, because you believe it is farther away at the horizon you perceive the moon to be much larger. Either way, your brain has been tricked.
The Ponzo Track
The theory that the objects in the foreground affect how far away we believe the moon to be can be comprehended by looking at Mario Ponzo’s railway track diagram in which two physically identical lines appear different sizes due to the perspective created by the tracks converging in the distance. The line at the top of the diagram appears wider than the line below because it seems to span a greater distance across the railway lines, which we wrongly perceive as parallel. We’re also reminded of the Father Ted episode when Ted explains to Dougal that the toy cows are ‘small’ but the real cows outside are ‘far away’.