Why does the moon look slightly bigger when it is near the horizon?

Answer 1

Its an optical illusion - if you measure it with instruments it is precisely the same, and the illusion fails if you take a photograph of it.

The origin of the illusion is still debated.

Answer 2

they've been arguing about this for two thousand years, and it's about time I got things straightened out. First of all, let me make it clear that the effect is an optical illusion. If you measure the moon with a ruler held at arm's length (a paper clip bent into the shape of a calipers will also work), you'll find it's always the same size no matter where it happens to be in the sky. If anything, the moon is slightly smaller at the horizon than it is at the zenith, mainly because it's 4,000 miles (the radius of the earth) farther away. (If you don't understand why this is so, draw yourself a picture.) Nonetheless, most people are convinced that, area-wise, the moon's at least twice as big when it's near the horizon as when it's overhead.

Numerous theories have been advanced to explain the ""moon illusion.'' At one point people thought it had something to do with the angle at which you hold your head and/or eyes while viewing, while others said it was caused by differences in the moon's brightness when seen at various locations in the sky (a notion first proposed in 1709). Both ideas have long since been discredited. The fact is that the illusion is dependent entirely on the visual cues provided by the terrain when the moon is near the horizon, and the lack of such cues when it's at the zenith. To prove this, try viewing the moon through a cardboard tube or a hole punched in a sheet of paper to mask out the landscape--the illusion disappears.

What's now called the "apparent distance" theory was first advanced by the Egyptian astronomer Ptolemy in the second century AD (I told you this goes back a ways). His explanation is a little confusing, but here goes: most people subconsciously perceive the sky to be a flattened bowl--i.e., objects near the horizon seem farther away than objects overhead, due to the abundance of intervening visual cues on the ground. Now, when we see an image of a certain size at what we believe is a great distance, we deduce that it's bigger than an image of the same size seen at what seems to be a lesser distance. (You might want to let this percolate for a minute.) So when we see the moon at the "distant" horizon, we subconsciously conclude that that it's "bigger" than when we see it a few hours later overhead, when it's "close." To put it another way, perspective--i.e., the march of visual cues to the horizon--makes the the moon look bigger than it does when it's just hanging in space.

This explanation is OK as far as it goes, but it's even better if we combine it with one propounded by psychologist Frank Restle. Frank reasons thusly: you judge the size of something by comparing it to the size of things around it. If it's surrounded by big things, it seems little. If it's surrounded by little things, it seems big. When the moon is overhead you judge its size against the vast expanse of the night sky (the stars are too small and faint to make any difference). Ergo, it looks small. When the moon is close to the horizon, on the other hand, it's usually bigger than many nearby objects (trees, houses, waves on the ocean). In addition, just after moonrise (when the illusion is most compelling), the moon's apparent diameter exceeds the distance from the moon to the horizon. Add in the effect of prespective, and the moon looks huge. That's all there is to it.

Answer 3

I scored a Best Answer for this the last time it was asked. It's an optical illusion. Near the horizon, you have objects which you can compare to the size of the moon, but when it is high in the sky, you don't. The comparison makes it look bigger.

Answer 4

Coz of refraction of relfected light rays, as the rays pas through more pathlength in the atmosphere before reaching the retina of the observer. Refraction bends the rays so that the size looks bigger for the observer

Answer 5

It's an optical illusion.

Answer 6

perspective, lack of visual references, [seemingly] greater luminocity