The night sky is the dark sky in deep twilight or at night . In astronomy , it is primarily understood to mean the starry sky, i.e. the night sky with little or no cloudiness and good visibility of the starry sky .
The term is limited in time by the period between dusk and dawn , that is, when the sun is more than 12 ° below the mathematical horizon of the location in question. At this low angle and clean air , the landscape horizon is only faintly visible, but a larger number of stars . However, complete darkness can only prevail when the sun is more than 18 ° below the horizon (end of astronomical twilight or beginning of astronomical dawn).
The transition from the brightness of the day to the darkness of the night is called twilight and is caused by the scattering of sunlight in the earth's atmosphere. In Central Europe it takes between 1 and 1½ hours, depending on the season, and is scientifically divided into three parts:
- Civil twilight (reading outdoors possible, between sunset and 6 ° depth of the sun)
- Nautical twilight (horizon and brightest stars visible, colors still partly recognizable; depth angle of the sun 6–12 °)
- Astronomical twilight (transition to complete darkness and night vision , depth angle of the sun 12–18 °).
Visibility at night
The difference in brightness between the day and night skies is around 1: 1 billion in clear weather, and a little less in cloudy weather . The human eye compensates for this great difference through the transition from cone to rod vision ; however, these very light-sensitive visual cells cannot see colors , only shades of gray . At night, color vision is therefore only possible with very bright stars and brightly lit surfaces - which has led to the saying “all cats are gray at night”.
Overall, the human eye can adapt to differences in brightness of 1: 100 billion, for which it takes about 1 minute (dark → light) or 15 minutes (light → dark). The latter transition occurs more quickly in youth than in old age, which is why older drivers , for example, are much more at risk from glare than younger ones. The last stages of the light-dark adaptation are already disturbed by a slight illumination (especially by light containing blue), which is why astronomers only use a very dull reddish light to look at star maps .
The night sky is never completely black, even in good weather - apart from a few cases in space travel . In addition to the moon and the clouds, etc. a. Dust particles in the air envelope - especially in the haze over larger cities - and so-called light pollution from street lamps or headlights pointing upwards (e.g. disco spotlights). Even in the best case (without any external lighting) there is a slight brightening of the night sky due to the so-called recombination lighting of the air molecules in the first hours of the night.
Other bright spots have astronomical causes, which include (ranked according to the size of the effect):
- the moon , which prevents any high-quality sky recording in the days around the full moon , but also affects it as a narrow crescent moon ,
- Aerosols in the higher atmosphere, for example Saharan dust and finely divided volcanic ash.,
- Northern lights (aurora) and glowing night clouds (thin ice clouds in the mesosphere ),
- temporarily the zodiacal light (fine dust in the ecliptic ),
- the light scattering of the brighter stars in the troposphere .
The brightening of the terrestrial night sky means that even with the largest reflecting telescopes, no stars fainter than about 23 m can be observed. This limit, which corresponds to a few candlestick strengths at a distance of the moon, can only be exceeded by space telescopes .
With bright photo lenses and highly sensitive films or digital cameras , you can expose up to a few hours in locations in the high mountains before the brightening becomes effective as a “gray veil” and the weaker stars are outshone by the image background. At the edge of a large city, this maximum exposure time is reduced to a few minutes. In the case of planar celestial objects, this restriction becomes much more noticeable, which is why in cities, for example, the glowing band of the Milky Way can only be seen very rarely.
The Bremen doctor and amateur astronomer Wilhelm Olbers made important considerations about the brightness of the night sky and the structure of the entire universe . His question as to why the night sky appears dark to us at all has come to be known as " Olbersian Paradox ".
The brightness of the night sky in a city is approx. 19 mag / arcsec² (4000 S 10 ). Under optimal conditions, it drops below 21.6 mag / arcsec² (370 S 10 ). The proportion of the airglow is 150 S 10 , that of the zodiacal light is 60 S 10 . Near the horizon, the starlight has to pass more than 40 air masses L and is almost extinguished. According to the estimate
you lose three size classes if you want to observe stars at 10 degrees above the horizon, corresponding to approx. 5 air masses.