Topic: Sky!

[tangerine dream]

[tangerine dream]

Thunder Bay last night...

[Angela]

Thunder Bay last night...

pretty! 

[nichi]

San Diego

[nichi]

[nichi]

Norway

[nichi]

[nichi]

[nichi]

[nichi]

[nichi]

[nichi]

[nichi]

[nichi]

[nichi]

the Shadow sets...



Interesting explanation here.

I've seen it when I fly back to Virginia. I don't know why, but it has some significance for me around which I haven't wrapped my mind.

Just before sunset, a low flat, dark blue band rises up from the eastern horizon. This is the earth shadow and it streches for nearly 180°. It is bounded above by the pinkish antitwilight arch and below by the horizon. The earth's shadow is best seen when the sky is clear and our line of sight is long. From a high elevation, the shadow appears sharper than it does from ground level. As the sun sets, the boundry between the earth shadow and the antitwilight arch rises in the sky and becomes progressively less distinct. With the disappearance of the antitwilight arch, the shadow blends smoothly with the deepening blue night sky. The earth shadow can be seen at twilight and sunrise most days, but it is not always blue.

For a shadow to be seen, it must be cast upon something. The earth shadow is is cast upon the atmosphere. Eventually the antitwilight arch fades to blue. Its strong color and brightness near sunset originate in backscattering by the relatively thick lower atmosphere, being illuminated by direct (reddened) sunlight. But as the twilight rises, so does the lower bound of the atmosphere, being illuminated by direct sunlight. Being higher and thinner, it scatters less red light and a point is reached where the strong bluish airlight, the same multiply scattered skylight making the earth shadow blue, becomes the dominant source of light reaching our eyes.

How can we see the earth shadow before sunset? Is it geometrically impossible? In fact, its dark bluish rim can be seen several minutes before the sun disappears. What the observer is seeing is the shadow of the transluscent atmosphere, behaving like a piece of smoky glass and casting its own indistinct shadow on itself.

Why does the sky overhead remain blue during the entire twilight sequence when virtually every other part of the sky has changed color? The earth's stratospheric ozone layer (about 12 km up) absorbs strongly at longer wavelengths, this filtering out the reds, oranges, and yellows, leaving the zenith sky blue.

Purple after sunset

Purple light can sometimes be observed in the west some 15 to 30 minutes after sunset. The reddish colored light of the evening sun is scattered again in higher altitude. After the sunset the sun illuminates aerosols in higher altitudes of 10 to 20 km above ground (lower stratosphere). Mie-Scattering on this small aerosol particles (typically 100 to 1000 nm) is responsible for the reddish-purple color of the late evening sky.
http://www.webexhibits.org/causesofcolor/14E.html