Twilight
When the sun has just set the brightness of the sky decreases rapidly thereby enabling us to see the airglow that is caused from such high altitudes that they are still fully sunlit until the sun drops more than about 12° below the horizon. During this time, yellow emissions from the sodium layer and red emissions from the 630 nm oxygen lines are dominant, and contributes to the purple-ish color sometimes seen during civil and nautical twilight.
After the sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source.
When astronomical darkness has set in the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs
Read more about this topic: Sky Brightness
Famous quotes containing the word twilight:
“Be near me when I fade away,
To point the term of human strife,
And on the low dark verge of life
The twilight of eternal day.”
—Alfred Tennyson (1809–1892)
“Love that had robbed us of immortal things,
This little moment mercifully gave,
Where I have seen across the twilight wave
The swan sail with her young beneath her wings.”
—George Meredith (1828–1909)
“It is possible to believe that all the past is but the beginning of a beginning, and that all that is and has been is but the twilight of the dawn. It is possible to believe that all the human mind has ever accomplished is but the dream before the awakening.”
—H.G. (Herbert George)