To understand this, have a look at the diagram below, the orange circles are exactly the same size. If you don’t believe me get a ruler and measure them! This is the Ebbinghaus Illusion.

When large blue circles surround the orange circle it appears smaller, and when small blue circles are surrounding it the orange circle seems bigger. The orange circle remains exactly the same size but it looks bigger and smaller depending upon its relationship to nearby objects.

Consider a cloudy sky. When you look directly up, the clouds seem bigger than those on the horizon. Instead of all parts of the sky looking the same distance from us, the higher up we look the closer it seems, almost as if the sky is squashed down on us.

Now imagine the orange circles are the moon, and the blue circles are clouds. You can see that when the moon is next to large clouds it would look small, and conversely when it is on the horizon near small clouds it would look bigger.

When we judge size against distance we subconsciously use a ‘size constancy’ effect. Our minds adjust for size and distance all the time and it works even when there are no clouds in the sky.

Now consider the moon. It is the same size and distance from you whether you look directly up at it, or look at it low on the horizon. However our minds think the sky is nearer to us above our heads (remember the larger clouds).

So when we see the moon low on the horizon (apparently far away) we subconsciously use a ‘size constancy’ adjustment and it looks bigger than reality. Similarly when the moon is above us, it appears to be nearer and therefore smaller than actuality. The difference in apparent size, especially with full moons, can be massive – but it’s just an optical illusion.

John McNally