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:
“The tide rises, the tide falls,
The twilight darkens, the curlew calls;
Along the sea-sands damp and brown
The traveler hastens toward the town,
And the tide rises, the tide falls.”
—Henry Wadsworth Longfellow (1809–1882)
“There’s no twilight in the tropics. Night falls like a curtain.”
—Waldemar Young, U.S. screenwriter. Erle C. Kenton. Dr. Moreau (Charles Laughton)
“Bid a strong ghost stand at the head
That my Michael may sleep sound,
Nor cry, not turn in the bed
Till his morning meal come round;
And may departing twilight keep
All dread afar till morning’s back....”
—William Butler Yeats (1865–1939)