Dark of the moon is darker than the sky1/27/2024 It was not until the 1920s that scientists were beginning to identify and understand the emission lines in aurorae and of the sky itself, and what was causing them. When physicist Anders Ångström examined the spectrum of the aurora borealis, he discovered that even on nights when the aurora was absent, its characteristic green line was still present. Sources of the night sky's intrinsic brightness include airglow, indirect scattering of sunlight, scattering of starlight, and light pollution. When the Sun's depth is more than 18°, the sky generally attains its maximum darkness. At astronomical twilight, the Sun is between –12° and –18°. At nautical twilight, the Sun's altitude is between –6° and –12°. At civil twilight, the center of the Sun's disk appears to be between 1/4° and 6° below the horizon. Twilight (both dusk and dawn) is divided into three 6° segments that mark the Sun's position below the horizon. During twilight (the duration after sunset or before sunrise until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required. During daytime, when the Sun is above the horizon, the direct scattering of sunlight is the overwhelmingly dominant source of light. The sky's brightness varies greatly over the day, and the primary cause differs as well. the Moon and light pollution) were removed from the night sky, only direct starlight would be visible. The fact that the sky is not completely dark at night is easily visible. Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light.
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