As the setting sun goes behind a cloud, you will often see rays of sunlight radiating in all directions. Why the sunburst effect? And if you trace the converging rays to their point of origin, it suggests that the sun is hovering above the cloud. What’s up with that? asks Patrick Bowman.
Whether we call them by their official name — crepuscular rays — or the much friendlier “sunbeams,” these shafts of light appear only under certain conditions. First, there must be enough particles — dust, water, or gases — in the air to scatter the light to our eyes. (If a sunbeam springs to life in your living room, thank airborne dirt and dog hair for the display.)
The second key ingredient in the recipe for rays is “shadow.” Clouds or mountains block some light, creating alternating columns of sunlit and shadowed air in the sky. Presto: rays.
To understand why rays of light from the Sun – traveling side by side — appear to converge to a point, just think about train tracks. Drive up to a train crossing, and you’ll see that the rails run parallel to each other. (The rails on standard train tracks are separated by 56.5 inches.) But as you cross a straight set of tracks and look into the distance, you’ll see the tracks appear to narrow.
In fact, at the horizon, the two rails appear to converge to a point. Meanwhile, trees along the track also appear to converge, shrinking as they march into the distance. Scientists (and artists) call this “perspective.” The point at which trees and tracks converge and dwindle to nothing at the horizon is known as “the vanishing point.” On the other hand, tracks and trees appear to spread out as they lead from the horizon to us.
Like rails on a track leading from a train terminal, light rays streaming from the Sun are nearly parallel. But because of perspective, we see the rays converge on or near the distant Sun, while seeming to fan out into the sky away from the Sun.
According to astronomer Philip Plait, the reason why rays can seem to converge above the Sun’s actual position is because of several optical illusions. One is that the sky is a curving bowl. Another is that some of the rays appear to be shooting straight up, rather than traveling over our heads. Processing the confusing visuals, the brain may perceive the rays as originating above the Sun’s disc.
Crepuscular means “twilight,” and while we see rays at other times of day, they’re especially likely to appear near sunrise and sunset. And if viewing conditions are right, with just enough reflecting haze, we may see rays covering the whole sky, from the setting or rising sun to the opposite horizon.
Above our heads, the rays will look roughly parallel. At the opposite horizon, they will re-converge to a point. (Thank that railroad track perspective again.) At this “antisolar point,” the rays appear to emanate from a hidden second sun.
See these anti-crepuscular rays over the ocean at http://apod.nasa.gov/apod/ap060917.html.
