7 vote(s)
JTony Loves Brains
5
Charlie Fish
5
GYØ Ben
5
Hemingway Kat
5
Loki
5
Jellybean of Thark
5
zeraphere zeraphere
Terms
(none yet)4 comment(s)
What is the green flash bit about in the instructions?
Mink,
Apologies if this is either too simple or too confusing. (Haven't got a clue what you do or don't know about optics.)
As sunlight enters the earth's atmosphere, it is diffracted and its direction changes very slightly. It's the same phenomenon that distorts the apparent position of an object viewed through the surface of a pool of water, but orders of magnitude smaller.
It turns out that the amount by which light is diffracted varies with the wavelength of the light. (There are good reasons for this, but it's a bit involved and something of a tangent.) Long wavelength red light gets bent the least. Shorter wavelength green light gets bent more. Blue light would get bent the most, except that it also gets scattered and sent off in random directions by air molecules and so we don't see much of it coming from the direction of the sun. (But, we do see a blue sky as a result.)
As the sun sets, first a truly straight line from you to the sun gets cut off by the horizon. But, you can still see the sun because its light is bent slightly from a straight line. Next, the less bent red light from the sun gets cut off by the horizon. Eventually, all the light except green is cut off - and for a moment, all you see is the green component of the sunlight.
At least, that's the story. If you actually grind through the numbers assuming typical atmospheric densities, it turns out that the angular separation between the red and green component of sunlight is quite a bit smaller than the human eye can distinguish. You need surprising atmospheric layering or some other weather related weirdness to explain the many stories of an occasional "green flash." (I've certainly never seen one.)
Remembering that "westernmost point in San Francisco" really means "westernmost point in San Francisco that you can reach" in this global game is important, otherwise lovely pictures like this would be lost to SF0. In other words (and to Frankenstein some old sayings together) San Francisco is where your heart is!