Sunsets are colorful, beautiful wonders to see the world over, and one of the most mysterious and rarest things to see in a sunset is the fabled "green flash." Even the most cursory everyday observation reveals that the Sun fades and turns reddish-orange as sunset approaches. This reddening is caused by Rayleigh scattering of light by molecules in the atmosphere.  The result is that the Sun looks red because so much blue light is removed from the line of sight.

   Astronomers use the term "air mass" to describe the Earth's atmospheric thickness in the line of sight. An object overhead at an angle of 90° is seen through one air mass, at altitude of 30° through two air masses, and so forth.> At sunset we see the Sun through the equivalent of about 38 air masses.  In a clear atmosphere the setting Sun is about 1/1000 its mid-day brightness, but even a moderate amount of dust or haze can reduce this figure further.

    A more detailed examination of the setting Sun shows that it also appears somewhat flattened and may show horizontal structure and banding. The flattening is due to atmospheric refraction, which raises the Sun's lower limb by about 35 minutes of arc and the upper limb by only about 29 minutes when the lower limb is touching the horizon.  The banding effects are due to layers of differing temperature and density in the air.

   The setting Sun's disk is made up of light of all colors. Green and blue light are refracted by air slightly more than red light, so the disk actually consists of a flattened red disk, with a yellow disk slightly above it, a green disk above that, and blue and violet disks at the top.  This phenomenon is called atmospheric dispersion. The vertical separation of red and green varies with conditions, but is typically about 1' at the horizon.

As the upper rim of the setting disk approaches the horizon, it begins to spread into a thin bar of light, then runs through the spectrum from orange to yellow, then pale green, and finally reaches a deep emerald color for two or three seconds. > Under favorable conditions a brief blue blob of light may be seen after this, but exceptional clarity of the air is needed.

   The explanation for the green flash involves refraction, scattering, and absorption, 1) refraction separates the solar images by color; 2) at just the right instant, the red image has set, 3) the yellow image is absorbed; and 4) the blue image is scattered away.  We are left with the upper limb of the green image.