I’ve touched upon this topic, also known as environmental optics, on a number of occasions in the past, usually highlighting one particular aspect, such as rainbows, twilight glows or solar sights. There’s much, much more though, including most everything under the Sun. Blue skies result from Rayleigh scattering short wavelengths of light preferentially off air molecules. This scattering is the same reason that sunsets are red. Add some volcanic aerosols or dust, and watch the display of fiery oranges and strange purple glows. And with just the perfect circumstances for refraction, the last bit of the Sun departs with the elusive green flash.

Optics in the outdoors is a big topic. Mainstream physicists have sections on their leather-bound journals devoted to it, but such papers are an odd fit at best. Atmospheric phenomena can be dissected in highly technical detail and explained by geometrical optics, electromagnetic theory and wave mechanics.  The result is either arcane mathematics or reams of numbers from a computer models. Such an understanding helps, but doesn’t always lead to a better appreciation of the grandeur of the sky.

The authors of several, now almost classic books, show a proper balance of physics, aesthetics and enjoyment of these phenomena. My favorite is: Light and Color in the Outdoors by M.G.J. Minnaert (1974, available as 1993 translation). Robert Greenler’s Rainbows, Halos, and Glories (1980, a new edition is available) is also a great resource. Finally, Aden and Marjorie Meinel, professional astronomers at Kitt Peak, have written their reflections in Sunsets, Twilights, and Evening Skies (1983).

Because we have little or no ability to detect polarized light (take off your sunglasses if you have them on), we lack a true appreciation for it. To correct this shortcoming in our visual experience, there are two good books with plenty of photos and experiments to try for yourself: G.P. Können’s Polarized Light in Nature (1985 translation, Cambridge) and David Pye’s Polarised Light in Science & Nature (2001, Institute of Physics).

Atmospheric phenomena reflect the same physical principles that telescope makers must battle: differential refraction in lenses, scatter off dusty optics, and diffraction from secondary spider vanes. These are big trouble to the telescope builder and observer pushing the limits of what can be seen. But in the same sky, we see colorful sundogs, glories and red rainbows at sunrise. So they do have an up-side.

Even better, these phenomena are likely the easiest to photograph and often the most colorful pictures you can take of the sky. Unlike prime-focus deep-sky astrophotography, there is no polar alignment, no tedious eyepiece guiding, nor any reciprocity failure to contend with. A point and shoot camera, hand-held, real film or digital, will do just fine. The secret, if there is one, seems just to be in the right place at the right time, camera in hand.

Now the fog has lifted, and it’s clearing already. Hmm… now cloudy again. Anyway, the forecast looks good for tonight, so I’ll keep my fingers crossed. Meanwhile, time to get the kids off to school.

One of Doug’s New Year’s resolutions is to always keep his camera handy to photograph atmospheric phenomena. Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. Please address comments and suggestions to dcmiller@dmv.com. This document was last revised on 9 February 2002. All text and images copyright © 2002 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.