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Why Moon on horizon looks big

The moon looks larger when newly risen than when it rides high in the sky. This is a well-known illusion, whose explanation has been given a new twist. With the Moon near the horizon, our visual system "assumes" it is at the same distance as trees or other horizon features. Therefore the system makes the Moon seem as large as an object at that distance should be (in relation to its surroundings) to produce an image within the eye the size of the image of the Moon. This is called scaling the image for distance. It makes the Moon on the horizon appear larger than when it is higher in the sky, where it seems to be at an indefinite distance with nothing around it to give it scale.

Such scaling for distance applies to any object, not just to the Moon, and has been known for a long time. What hasn't been realized is that the Moon represents the limiting case. As seen from Earth's surface, the size of its image is about half a degree. This turns out to be the cutoff point for distance scaling, according to John Ross, B. Jenkins, and J. R. Johnstone of the University of Western Australia. Images larger than this are scaled for distance -- that is, the corresponding objects appear to maintain size as they get farther away. Objects that form images smaller than about half a degree, however, simply appear small, whatever their distance.

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Reporting their research in Nature, the Australian scientists explain: "Aeroplanes, after taking off, maintain object size and then, at a certain point , look small. The change occurs when image size shrinks to half a degree. Similarly motorcars, lorries, trains and houses, seen from the air, look small, like toys, when their images subtend less than half a degree. As a plane comes in to land, a passenger may observe the sudden transition from small to large occurring for each object as its image size breaks the half-a-degree barrier."

Testing this on the Moon, the scientists reduced its image by one-third by looking through the "wrong" end of a telescope. The moon high in the Sky seemed to shrink by the expected amount -- that is, by one-third. But the horizon Moon shrank two to three times. As its image became less than half a egree, it was no longer scaled for relative size and thus appeared no larger than the Moon seen higher up.

The scientists say this suggests that our visual system handles images in different ways -- "the large [image] being subject to automatic correction to take account of distance, and the small giving absolute size information, which is not corrected for distance."

Since the Moon is slowly moving away from Earth (due to tidal effects) and its image is consequently growing smaller, the time will come when there will be no Moon illusion -- no large, luminous, full Moon disc rising at dusk. So long, Harvest Moon.


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