Why do sunbeams appear to spread out?

25 October 2011

Question

Distance of sun from Earth.

I was out, walking my dog this morning, just as the sun came up. There was a thin cloud cover, but the sun's rays were coming through. I noticed that the light rays came in a fanned pattern. If I trace the rays back to a point of origination, I determine that the sun is maybe a mile up in the sky.

Either they've been lying to us all these years as to how far away the sun is, or something else is going on here.

Why are the light rays fanned out? Shouldn't they be more parallel if the sun is 93,000,000 miles away?

Thanks.
Ken Silva
Phoenix, AZ
USA

Answer

The correct name for this phenomenon is "crepuscular rays". It refers to the apparent spreading out from behind a cloud of sunbeams or shafts of sunlight when you view them from the Earth's surface.

It is, of course, an illusion. The Sun's rays are not issuing from behind a cloud and spreading out towards you: the Sun is so far away that the light rays reaching the Earth are effectively parallel. Instead, this "trick of the light" is caused by perspective. That is, things farther away look smaller. This is why train tracks appear to narrow, or converge, into the distance.

So when you look skywards and see sunbeams coming through a cloud, because the light patches at greater distance appear closer together, making the shafts of light look narrower.

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