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:
“I’ve been in the twilight of my career longer than most people have had their career.”
—Martina Navratilova (b. 1956)
“But we still remember ... above all, the cool, free aspect of the wild apple trees, generously proffering their fruit to us, though still green and crude,—the hard, round, glossy fruit, which, if not ripe, still was not poison, but New English too, brought hither, its ancestors, by ours once. These gentler trees imparted a half-civilized and twilight aspect to the otherwise barbarian land.”
—Henry David Thoreau (1817–1862)
“The eastern light our spires touch at morning,
The light that slants upon our western doors at evening,
The twilight over stagnant pools at batflight,
Moon light and star light, owl and moth light,
Glow-worm glowlight on a grassblade.
O Light Invisible, we worship Thee!”
—T.S. (Thomas Stearns)