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August 18, 2005
Last night I looked out of the window and the moon looked massive on the horizon. Is this because of some kind of "Honey I blew up the kid" mischief or does it like to sneak up on us to remind us that it's there?
You may have guessed that it's neither of these – though the moon does have an elliptic orbit.
Previously a popular held belief was that dense air near the ground refracts the light from the moon acting as a magnifying glass. Scientists since have dismissed this as utter bollocks – in a manner of speaking.
To see that the moon in fact stays the same size wherever in the sky it is measure it. The best way is to roll up a newspaper and stick it against your eye like a telescope. Tighten it so the moon fits just inside and sellotape it in place. Whenever you look at the moon through the paper, it will fit snugly inside.
Okay, so it's an optical illusion of some form, but what's going on? Some of the supposed best explanations on the internet are written by very clever and smug scientists who are intent on making sure everyone who stumbles across their work realises how very clever and smug they are. Unfortunately this has two effects: firstly, it's extremely boring. And secondly, it doesn't make any sense. Unless you yourself happen to be a very clever and smug scientist.
This is the basics though:
The Ponzo Illusion . Viewers of such TV programs as How 2? will be familiar with this sort of trick. There's a school of thought that suggests the moon's size appears larger because of houses and trees acting as the lines. It's a neat explanation but it falls short. People in aeroplanes observe the same trick with the moon and as a rule they don't have many houses or trees to gauge the moon against.
The flattened sky . The thinking here is that we perceive the sky to be dome shaped. So we expect objects on the horizon to be further away – in the case of things in the sky such as birds or planes they actually are. So we expect a moon on the horizon the be (significantly) further away than one in the sky above us. This leads our minds to compensate so we perceive the moon to be bigger. This is neat and tidy, but the big brains prefer a similar but slightly more technical explanation…
Oculomotor Micropsia and Oculomotor Macropsia, which to my immense disappointment are not spells performed by Harry Potter. When we look at a moon in the sky we have no other objects to gauge it's distance. It may aswell be a few metres away, which our silly little human brains believe it is. This is where oculomotor micropsia sets in, making the moon look smaller. When the moon is on the horizon we can tell it's further away by gauging it against trees and houses. Because of this we have oculomotor macropsia and we perceive it to be larger.
Quite a lot of gobbledy gook I'm sure you'll agree, personally I'm quite happy with looking up at the sky and thinking "oo, pretty".