why is the sun so big when it rises?

[Dak]

is it something to do with light 'blurring' (diffusing?) as it passes through the atmosphere at an angle?

[D H]

Nope. Just something to do with your eyes (and your mind).

 

The effect is called the Moon illusion; you see the same thing happen with a full moon. If you took a series of pictures of the Sun or the Moon you would see that the change in size is purely a matter of perception.

 

You can google for Moon illusion and get a host of different explanations.

[DJBruce]

The best theory I have heard says this is cuased by the fact that when it rises your mind compares it to objects you see like buildings in the background and this cuases your mind to play tricks on you thinking its bigger.

[antimatter]

Perhaps we are just viewing it a different angle than we normally view it?

I'm not entirely sure myself, though I have thought about it before.

[D H]

The best theory I have heard says this is cuased by the fact that when it rises your mind compares it to objects you see like buildings in the background and this cuases your mind to play tricks on you thinking its bigger.

That explanation fails because sailors and airplane pilots also see the rising or setting Moon (particularly a full Moon) appear to be large.

 

Photographic proof that it is an illusion in this overlaid sequence of photographs of the Moon rising over Seattle:

[swansont]

As D H suggests, it's not the direct comparison of items. It's sometimes called the bowl effect — the sky is not really a hemisphere, it's more of a flat-bottomed bowl. Clouds overhead are within a few miles of you, but the the ones on the horizon can be a hundred or more miles away, and you can see clouds over ground that is invisible to you because of the curvature of the earth. That makes us bad at judging distances in these situations. We interpret the moon's distance on the horizon differently than the moon's distance overhead. Taken in conjunction with the Ponzo illusion (if you think something is far away, you interpret it as being physically bigger), that's the thought for why the moon or sun looks bigger.

 

(Refraction does make it a tiny bit bigger, but that doesn't explain the perception)

[Pangloss]

Yah that refraction also causes the sun and moon to appear to rise before they actually do, and set after they already have, which I've always thought was really interesting. If memory serves, the refraction level is something like a minute of arc at the horizon, or about the width of the sun and moon, so when you see them touching the horizon they're actually below it.

[D H]

(Refraction does make it a tiny bit bigger, but that doesn't explain the perception)

Refraction makes the moon's image smaller, not larger. Another confounding factor is that the Moon is further away from a viewer on the surface of the Earth at moonrise/moonset than it is when the moon is at its zenith. Combining these two effects makes the moon's image subtend a solid angle that is 2% smaller at moonrise/moonset than it is at its zenith.

[swansont]

Refraction makes the moon's image smaller, not larger. Another confounding factor is that the Moon is further away from a viewer on the surface of the Earth at moonrise/moonset than it is when the moon is at its zenith. Combining these two effects makes the moon's image subtend a solid angle that is 2% smaller at moonrise/moonset than it is at its zenith.

 

D'oh. I was thinking of the position, as Pangloss mentioned. The sun gets flattened, along with some other neat effects that are possible.

 

http://mintaka.sdsu.edu/GF/explain/simulations/ducting/duct_intro.html