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Night and day January 11, 2008

Posted by David Corney in Uncategorized.
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Blogging on Peer-Reviewed ResearchAlthough I’ve been researching vision and creating synthetic images for a while now, I’d never really thought about night-time illumination. Well, not beyond thinking, “It’s dark at night!” I suppose. But then I read a paper [1] by Javier Hernandez-Andres and his group in Granada about crepuscular and nocturnal vision, and learned that natural light at night is a lot more complex (and interesting) than I thought. During the day, all light come from the sun or from skylight, which is just scattered sunlight. But at twilight and through the night, there are many and varied light sources.

After the sun passes below the horizon, it still lights up the sky for a while so that’s one source of light. Then there’s moonlight, which is a direct reflection of the sun and has a very similar spectrum to daylight, at least for a high and full moon. And there’s starlight, which has a spectrum roughly like daylight but fainter and with four distinct spikes around the yellow / red region. Then illumination starts getting really exotic. There’s “airglow”, which was first (officially) noticed by Anders Ångström (he of the unit) in the mid-19th century. It consists of various light-emitting molecular processes in the upper atmosphere, which produces a faint blue-ish glow across the sky. Then there’s “zodiacal light”, which was noted by Cassini (he of the Saturn orbiter) in the 17th century. It consists of sunlight bouncing off scattered cosmic dust between the planets of our solar system, so again it has the same spectrum as sunlight, albeit fainter. And apparently, the very dark blue sky seen during late (“nautical”) twilight is that colour because of ozone absorption, and not (just) due to sunlight scattering effects. In other words, it’s not just “blue sky but a bit darker”, but is blue for a different reason.

The final nocturnal light source Hernandez-Andres et al. mention is anthropogenic light – light pollution. This varies enormously across space and time of course, with a strong yellow/red shifted spectrum suddenly appearing whenever a million streetlights click on at dusk, along with car headlights, office lights, advertising hoardings etc. etc. Scientists are now realising that many nocturnal animals, including some moths, rely on very subtle colour cues for foraging and mating, just as diurnal animals do. What effect light pollution is having on these creatures seems to be unknown, but presumably it forms a strong selection pressure, at least near built-up areas. Sounds like a ripe area of future study…

Johnsen, S. (2006). Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor. Journal of Experimental Biology, 209(5), 789-800. DOI: 10.1242/jeb.02053

Other links:

http://en.wikipedia.org/wiki/Airglow

http://en.wikipedia.org/wiki/Zodiacal_light