Figure One

Science. Communication. Community.

Moon, Shadow, and Science at 30,000 Feet

An online discussion serves up optics and illusions, as scientists debate a “mysterious light spot seen from the air.”     

by Amanda Alvarez             


A rainbow and an airplane shadow intersect in another atmospheric optical effect called a ‘glory.’ Image credit: flickr user Arenamontanus

Inspiration for science – and science writing – can crop up in unusual places. Normally I might not give much attention to an email with the subject line “Mysterious light spot seen from the air” – sounds like UFOs, right? This message, however, came via the color and vision mailing list that, as a former vision scientist who has an abiding interest in light and perception, I still subscribe to. The sender described a bright spot on the ground, trailing the flight he took from Taipei to Shanghai on a moonlit night this July. What was its cause, he asked?

The vision and optics community responded with dozens of possible explanations. Rice fields were reflecting the point source of the moon, said one. An Arago spot, a bright area within a shadow resulting from Fresnel diffraction that is an important demonstration of the simultaneous particle and wave nature of light,1 was suggested by others. A few proposed The Glory of the Pilot (pictured above), a shadow cast on water droplets seen from on high. Yet others maintained that they “too have seen strange things out of plane windows, sometimes before the drinks trolley has been round more than once.”

Though there was no definite consensus reached by the listserv community – and why should there be, this is science! – the explanation of specular reflection from wet pavement below the plane seemed to gain traction. With this hypothesis, the asphalt acted like a mirror for the moon, perhaps with some contributions from the metal body of the plane. Many on the list chimed in with details of the viewing geometry, the weather conditions that night, and the elevation and angle of the moon, and threw around educated guesses and personal anecdotes from both piloting and traveling in aircraft. Reading the responses felt like receiving a master class in atmospheric optics and space psychophysics, but also in reasoned debate, conjecture, and the best spirit of scientific inquiry. The great thing, as one respondent put it, is that anyone can re-run the experiment next time they are on a plane, and add their own data point to the mix.

The moral of the “mysterious spot”? Even if you’re already a scientist, just looking around with an inquisitive eye can turn up unexplored phenomena and experimental ideas. For science writers, leads can come from obscure sources, like the niche vision scientists’ listserv (I now fully expect a glut of stories on visual illusions, blind spots, and other optical arcana). For anyone looking to write those stories, a mailing list subscriber pointed the community to a book on the subject: “Science From Your Airplane Window.” There’s even an entire conference devoted to “light and color in nature,” which sounds like a great source for stories with captivating visuals. For this scientist-turned-writer, the “mysterious spot” served as a reminder that scientific discourse frequently happens outside the pages of peer-reviewed journals – and those discussions might actually be a lot more illuminating.

1 This license plate is one vision scientist’s homage to the duality of light.


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This entry was posted on September 17, 2013 by in Physics, space, Uncategorized and tagged , , , , , .
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