Why do the rays of the sun appear to radiate from a point?January 21, 2011 12:56 AM   Subscribe

Why do the rays of the sun appear to radiate from a point?

When you look at a sunset, or the sun shining through clouds, you appear to see sunbeams radiating from the small dot of the sun. Almost as if the sun was a small object, inside Earth's atmosphere. However, the sun is massive and distant, and its rays hit the earth effectively in parallel. Is what I'm seeing due to refraction, and the rays are actually diverging, or is it an optical illusion? Or something else?
posted by snarfois to Science & Nature (8 answers total) 2 users marked this as a favorite

optical illusion (parallel lines projected onto a sphere)...they're called crepuscular rays and if you're lucky you might even see anticrepuscular rays...
posted by sexyrobot at 1:13 AM on January 21, 2011

Response by poster: Thanks for the speedy answer! The Wikipedia article is fascinating. Although its explanation seems different to yours: simply linear perspective. Of course: parallel train tracks appear to radiate from a point at the horizon. Head-slappingly simple.

(The "parallel lines projected onto a sphere" explanation initially also sounded plausible to me, but it's probably not true. I agree parallel beams mapped onto a sphere would seem to radiate from a perspective inside the sphere, but I don't think that's what we're seeing here. But would that perhaps explain why contrails sometimes seem to curve?)
posted by snarfois at 1:46 AM on January 21, 2011

Best answer: Yea, I'd have said just a plain old trick of perspective.
posted by londonmark at 4:58 AM on January 21, 2011

The rays are parallel over half of the earth's circumference.Due to your vantage point perspective is going to play into it. The same question could be asked of railroad tracks - why do they recede in the distance when they are obviously parallel.
posted by JJ86 at 5:56 AM on January 21, 2011

More about atmospheric rays and related phenomena can be found here, including lots of pictures.
posted by TedW at 6:34 AM on January 21, 2011

Think of the rays as traveling towards you rather than down towards the ground. Even the rays that are illuminating the distant horizon are hitting it very obliquely as they travel towards you. The sun is not above the horizon but unimaginably far beyond it (and train tracks are all coming at you from it).
posted by bonobothegreat at 7:34 AM on January 21, 2011

Best answer: Yep: Perspective. The sun is the vanishing point.

Imagine you're looking through a long tube. The other end of the tube looks smaller than the end you're looking through, because it is far away. Imagine the inside of the tube is marked with parallel lines running lengthwise between the two openings. These lines, despite being parallel, appear to radiate from the opposite end of the tube despite being parallel.
posted by Sys Rq at 9:27 AM on January 21, 2011

Of course the light from the sun isn't even parallel, the sphere of the sun being so much larger than Earth and each point of light on the sun's surface radiating in all directions outward, but let's drop that complexity.

The key here is not that the sun is effectively a point. That's not the point that matters. The one that matters is the imaginary point in the perspective diagram that you occupy. (Your mind is complex, too, and actually processes the vision input from two separate eyes into one view, while at the same time maintaining both inputs for purposes of orientation and balance....) Anyway, for simplification purposes, you are the point that matters, and for any given parallel lines, you occupy a space somewhere between them or on only one of them. Thus, the other line is not where you are, creating perspective. Your perception of distance brings them closer and closer together even if they are not actually converging. Theoretically, you would perceive two infinitely parallel lines as converging, but with a device sufficient to distinguish their sources you could still see that they do not. Your eyes just aren't that awesome.
posted by dhartung at 11:40 AM on January 21, 2011

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