Sunset


Click on image for larger version.

Not unlike an edifice of some ancient culture, this mountain marks the turning point for the Sun in its cyclic dance from North to South and back North again during the course of year. (actually, from Kitt Peak it just misses to the South, as this image was taken on the 10th after the partial solar eclipse).

More importantly, demonstrated here are both the semantics and subtlety of the "green flash." Of course, the progenitor/cause of the green flash is the "green rim" of the setting sun. Due to atmospheric chromatic dispersion, the blue/green image is on top and a red image is refracted to below the mean position of the Sun (see Livingston, "Color and Light in Nature" page 48) Thus, under clear conditions with a low horizon it is possible to see the green rim quite often *with optical aid* (while this can be done with unaided eyes, it is not easy for the uninitiated).

Only with inversion layers and more complicated atmospheric densities can the image of the green rim be refracted higher in the sky- resulting in the green "flash". The flash is more rare (not captured here) and, as you might expect, is often seen over the ocean.

Unfortunately a dazzling setting sun also happens to leave an afterimage on the retina that is green in color as well- which often confuses people into thinking they have seen the green flash (where they have just blinked).

So, no comic book heros to be seen in this animation- merely a very subtle blue/green rim of the setting sun- with the last pieces of green (rim) to remain seconds before the sun sets. I have seen *much* better with my eye (and binoculars)... but this is my only attempt at photographing what I have seen for years. The change in background brightness is due to the film's response under changing light conditions (the exposure time was the same).


Equipment

8in Celestron telescope operating at f/6.3
Pentax camera with Fuji 800 film.

Minimum credit line: Adam Block/NOAO/AURA/NSF

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Updated: 6/25/2002