Simulation of the Textbook Green Flash


Here's a magnified view (3 times larger) of the last 8 seconds of this textbook sunset, displayed in real time (this is about what one sees through good binoculars, or the viewfinder of a camera with a long telephoto lens):

animation of a textbook green flash

Now we see the green rim distinctly. And, as the orange part of the Sun disappears below the horizon, the green rim is indeed left for a second or so, making a (very feeble) green flash as it fades away. The trouble is, this flash is so puny it can only be seen through binoculars, not with the naked eye.

For a look at a real green flash, we must use a different atmospheric model. The classical example is the convectively-unstable surface layer produced by a sea surface warmer than the air above it. Simulations for such a model are shown on the next pages.

Remarks on the Green Rim

The rim shown in this simulation is far too bright, compared to the disk of the Sun. Actually, the real green rim is seen best about 5 to 10 minutes before sunset, when the Sun is still 3 or 4 diameters above the horizon. Closer to the horizon, the rapid increase in extinction (in particular, atmospheric reddening) makes the green fade so much faster than the red that it becomes practically invisible.

Still higher in the sky, where the extinction is even less, the rim appears blue. The blue upper limb of the Sun was first noticed by Pierre Bouguer in 1748, when he invented the modern form of the heliometer. Bouguer correctly identified the blue upper limb and the red lower limb as due to atmospheric dispersion; so perhaps he qualifies as the first person to recognize and understand a green-flash phenomenon (though the colored limbs on Jupiter were noticed, but not understood, by Delisle in 1715.)

I now have a realistic simulation of the red and green rims that correctly includes extinction and limb-darkening. But it isn't yet possible to present it as a movie; only selected snapshots are possible at the level of detail required to show the rims realistically.

© 1999 – 2002, 2005, 2006, 2012 Andrew T. Young

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