question about starlight hitting the Earth's atmosphere

[SRH]

Why do stars and even reflected light from the Moon appear to form a cross when they hit our atmosphere?
The light is not solely along 2 axis, but it is much more dominant along 2 axis.
I would think that given a spherical/curved atmosphere, the light would appear more as concentric circles rather than a cross.
Why 4-points and not 5, 6, or infinite?

Thanks.

[tusenfem]

you mean when you take a picture?

[SRH]

nope - just when you look up at the open sky.

I am in Florida and twice last week, the Moon made a cross in the night sky.
One night it was fairly head-on (I think it was a full moon), and the other night, it looked more like a diamond (the cross was at a slight angle).
I took off my glasses to make sure it was not an effect from my lenses, and it was still there.

[SRH]

I understand that this is a picture, but it looked like this with the naked eye...

http://3.bp.blogspot.com/_T9G6u_uDqw...s1600/Moon.jpg

[SRH]

Looking around the internet, it is explained as:
1. being behind a screened-in porch (which I was and never thought to go outside)
2. ice crystals in the upper atmosphere.

If it happens again, I will go outside.

[ctcoker]

You saw diffraction of the light through your porch screen. It was a cross because the openings in the screen are squares. If they'd been circular, you would've seen a circular ring around the Moon.

[SRH]

Thanks.