MadSci Network: Astronomy

Re: Sunbeams show aparent sun position as aprox 2x cloud hight, Why?

Date: Mon May 1 14:39:06 2000
Posted By: Philip Plait, Astronomer/Programmer
Area of science: Astronomy
ID: 956359027.As

Seeing the setting Sun cast out rays is one of the most ethereally pleasing aspects of skywatching. I've seen it hundreds of times, and it always makes me stop and look.

The rays really do appear to diverge: they are close together near the Sun, then spread out overhead. However, this is an illusion. The rays are almost exactly parallel. They only appear to diverge because of perspective. It's exactly the same thing that makes a long straight road (or railroad tracks) appear to converge in the distance. Those clouds near the horizon breaking up the sunlight into rays are hundreds of kilometers away. When the rays pass overhead, they are only a few kilometers over you. When they are nearby, they look parallel (like the sides of a road or the track on a railroad track). When they are far away they converge.

Remember, this is coupled with the illusion that the sky is a bowl shape. Near the horizon, the rays look like they are heading straight up, but that's an illusion too! They are headed in a direction over your head, but your brain interprets that as being straight up, since you are seeing this near the horizon. That is most likely why it appears to you that the beams do not converge at the Sun's location in the sky. They must converge there, of course, but they appear not to because of the combination of these two illusions. It's similar to the illusion of the crescent Moon not pointing directly at the Sun; the sky's apparent curvature fools your brain and throws off your perception.

Having said all that, I'll add that the individual rays are not truly parallel. They would be if the Sun were a point source, but it isn't. The Sun is roughly a half degree across, so the edges of the rays are not all coming from exactly the same direction. this affects each individual ray, making it fan out just a bit. The rays as a set are parallel, but each ray is a little bit fan-shaped.

Incidentally, these are called ``crepuscular rays''. Sometimes, if there is enough junk floating in the air, you can actually see them cross the entire sky, where they reconverge at the point on the horizon opposite the Sun. I have seen this a few times myself, and it's amazing. If you ever see it, you can surprise your friends by calmly saying, ``Oh yes, those crepuscular rays are converging at the anti-solar apex.'' You may want to practice this line a bit. ;-)

An excellent description of this can be found at Tony Demark's Crepuscular Rays page. You can also just do a web search on 'crepuscular rays'' and see what that nets you.

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