The sky is green, the night is dark - as simple as possible. This photo was done at Bryce Canyon, Utah. Canon 60Da, EF16-35mm f/2.8L II USM, 30.0 sec; f/3.5; ISO 6400
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| Green Sky Utah Airglow Astrophotography Canon 60Da |
The natural moonless night sky is dark but not black; it is not even blue (no scattering light from sun and moon). Earth atmosphere emits green, red, orange, yellow light- Airglow!
About 60% of the natural brightness of the night sky at zenith is made up of Airglow. Maximum brightest usually appears at about 10-15 degrees above the horizon, because of the greater the depth of atmosphere at this angle. Airglow is not always uniform. Gravity waves may create considerable variation of Airglow brightness.
You can see Airglow only in real dark sky, but naked eye could not recognize colors. Photos with long exposure able get the colors of Airglow.
The usually dominant color of airglow is green originated from molecular oxygen at 90 to 100 km high. Orange and reddish hue caused by atomic oxygen in 150-300 km above the ground, Yellow light comes from sodium atoms in a layer at about 90 km.
