The Green Flash
Sometimes, when Venus is crescent in the evening sky, if you see it through a
telescope with totally color corrected
optics (and if the image is
right side up), you'll see it as red on the bottom and blue on the top. That's because if
Venus is crescent in the evening sky you'll see it not far from the western horizon. And since the light
which you then
see from Venus comes into the Earth's atmosphere at
a slant, the colors get dispersed as they would going through a prism.
And
the closer Venus gets to the horizon, the greater the color dispersion. However, if you want to see Venus without
the color dispersion, you can use an ordinary binocular eyepiece with an uncorrected
field lens. Then, if you push
the image higher in the eyepiece field,
the chromatic aberration of the field lens will clean up the chromatic
aberration
of the Earth's atmosphere.
Now this color dispersion affects what you see when you watch the setting Sun. It does the same thing
to your image of the
Sun that it does to your image of Venus, low
on the western horizon. It puts the red on the bottom and the blue on the
top. And if you get the opportunity to watch the Sun go down in very clear weather, if you get to see it looking
yellow
rather than red or orange, you may see an interesting sight.
You may see a sudden color change as the last speck of the
Sun's
disk dips below the horizon. The red edge of the Sun's image goes down first, and the trailing edge is blue. This
sudden color change is called the "green flash", not because it flashes
brighter but because the color change is sudden.
And it is usually seen as green because the blue is
overwhelmed by the yellow of the sunset. Only rarely is it
seen as blue.
