Air thins with increasing altitude, but the thicker atmosphere at the bottom wraps light along the Earth's surface. This makes things near the horizon appear somewhat higher. Although stars above 45° altitude are affected by less than an arcminute (1/60 of a degree), light appearing at the horizon actually comes from over a half degree below it. The sun sitting there appears squashed when, in fact, it is geometrically below the horizon. Each day we get a bit more daylight from this effect: sunrise is a few minutes earlier, and sunset a few minutes later. And were it not for haze and atmospheric extinction at the horizon, we would see an additional half-degree strip around the sky, an area roughly the size of the constellation Corvus, the Crow.

But textbook formulae fail within a few degrees of the horizon: the atmosphere is no longer smooth layers that bend light gradually. There light finds its way through layers of air differing in temperature, pressure and humidity, all of which affect refraction. Light rays here take unique and complicated paths, and objects appear distorted, irregular in outline. Light rays bounce between layers and flip over: blue sky above can be reflected below.

A mirage is thus formed: it is a multiple or partially inverted image formed by various ray paths and reflections. I have found mirages most prominant on the coldest of winter days looking out over relatively warm coastal waters. Ships and distant buildings on the shore take on a bizarre symmetry with their tops flattened and reflected below them.

Additionally, refraction is dependent on the wavelength, and this differential refraction is responsible for the oft-seen colorful sparkle of low, bright stars. In the extreme, this produces the most dramatic and one of the rarest effects of atmospheric refraction: the green flash of the setting sun. This is a real effect, but certainly not a common occurrence: I have seen my share of sunsets, but observed the green flash only once—over the Pacific Ocean from La Jolla, California. A recent article in Sky & Telescope (December, 2004, pp. 97-99) provides more details.

Readable explanations of these phenomena can be found in Marcel Minnaert’s Light and Color in the Outdoors, the classic book on atmospheric optics. There are many excellent web sites where you can find striking photographs: Les Cowley's Atmospheric Optics and Harald Edens' Weather Photography are two of the best. Andrew Young's green flash pages are surprisingly comprehensive, including links to as much mathematical analysis as you might want. With so much happening to sunlight grazing the Earth, take the time to appreciate it. You’ll find a lovely sunset all the more enjoyable.

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 24 October 2004. Text and images copyright © 2004 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.