The sun has just set behind us, and it’s six degrees below the horizon. Most of the plume is in the “earth shadow,” not touched by direct sunlight. As you follow the plume upward into the higher altitudes, it catches the light of the setting sun, first red, then orange, yellow, and finally white as you go miles above the earth.
The red colors come from sunlight that has traveled through the longest stretch of the earth’s atmosphere. The color of the sun itself goes through a similar color progression as it sets. Sunlight traveling on a near tangent to the surface of the Earth has had the most blue light scattered or removed from each ray, leaving the red color.
The moon is a few hours from full, and is therefore almost 180 degrees opposite the sun at the antisolar point. The plume is casting a shadowbeam through the illuminated atmosphere. That shadow points directly toward the antisolar point. You can think of the dark line as a slice of unilluminated vapor seen edge-on. If that slice were completely unilluminated, we'd see stars through it.
It’s a good reminder that the light blue color of the sky is merely “the blue dome of the sky itself is really a semitransparent film of air interposed over the blackness of space.”
------
(These points are further explained and illustrated in Color and Light, page 176, 180, and 190).
Image from Atmospheric Optics.
Previously on GJ: Shadowbeams
