Night sky

The dark sky in the deep twilight or at night is called night sky. In astronomy, it refers mainly to the starry sky, that is, the night sky with little or no cloud cover.

The term is limited in time by the period between the astronomical dawn and dusk, that is when the sun is more than 12 ° below the astronomical horizon of the place in question. At this low angle and pure air, however, the landscape horizon is hardly to be recognized, a larger number of stars. Complete darkness reigns, but only when the sun is more than 18 ° below the horizon is (end of astronomical twilight ).

The view of the sky a few hours before or after dusk is often referred to as the morning or evening sky.

Twilight

The transition from the brightness of the day dark of the night is called twilight and is caused by the scattering of sunlight in the atmosphere. It lasts in Central Europe depending on the season 1-1 ½ hours and is divided scientifically threefold:

  • Civil twilight ( reading outdoors possible between sunset and 6 ° low angle of the sun )
  • Nautical Twilight (horizon and brightest stars visible, colors still partly visible, low angle of the sun 6-12 ° )
  • Astronomical Twilight ( transition to full darkness and night vision, low angle of the sun 12-18 °).

Visibility at night

The difference in brightness between day and night sky on a clear day is about 1: 1 billion, on cloudy days a little less. This large difference is similar to the human eye by the transition from cone to rod vision from; but these are very light-sensitive photoreceptor cells can not see colors, only shades of gray. At night, therefore, a color vision is possible only in very bright stars and in brightly lit areas - which " are at night all cats are gray " to the spell has led.

Overall, the human eye can adapt to differences in brightness of 1:100 billion, what it requires, however, about 1 minute (dark → bright ) or 15 minutes ( bright → dark). The latter transition goes in youth faster than employment in old age, which is why, for example, older drivers are much more vulnerable to glare than younger ones. The final stages of the light-dark adaptation are already a slight lighting (particularly blue -containing light ) is disturbed, which is why astronomers use for viewing star charts only a very faint reddish light.

The night sky is also in good weather never completely black - apart from a few cases in space. Brightening effect in addition to the moon and cloud cover, inter alia, dust particles in the air envelope - especially in the smog over major cities - and the so-called light pollution by upward street lamps or headlamps ( for example Disco radiator). Even in the best case ( without any external illumination ) there is a slight brightening of the night sky caused by the so-called Rekombinationsleuchten of the air molecules in the first hours of the night.

Other spells have astronomical causes, including ( ranked according to size of the effect ) include:

  • The moon, the full moon to prevent any high quality sky inclusion in the days, but it also affects even as a narrow crescent,
  • Aerosols in the upper atmosphere, for example, Saharan dust and finely divided volcanic ash.,
  • Northern Lights ( Aurora) and noctilucent 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 effect that no stars can be less than about 23m also observed with the largest telescopes. This limit, which corresponds to some candlepower in Moon distance can be exceeded only by space telescopes.

Sky shots

With bright camera lenses and high -speed film or digital cameras you can expose to a few hours on sites in the high mountains before the brightening as " gray veil " to take effect and the fainter stars are outshone by the image background. At the edge of a big city, this maximum exposure time is reduced to a few minutes. For planar objects in the sky, this restriction is even more distinctly felt, which is why, for example, the bright band of the Milky Way can be seen only very rarely in cities.

Key issues for the brightness of the night sky and the structure of the entire universe presented in 1800 at the Bremer physician and amateur astronomer Wilhelm Olbers. His question, why us, the night sky is dark at all, under the name " Olberssches paradox" become known.

Brightness

The brightness of the night sky in a city is about 19 ​​mag / arcsec ² (4000 S10). Under optimal conditions it drops below 21.6 mag / arcsec ² (370 S10). The proportion of Airglows is 150 S10 of the zodiacal light at 60 S10. Near the horizon the starlight must be more than 40 L air masses pass and is almost wiped out. According to the assessment

Lose three size classes, if you want to watch star already at 10 degrees above the horizon, corresponding to approximately 5 air masses.

The surface brightness of the sky is measured with a Sky Quality Meter.

Air pollution Type color Adaptation (eye) Haze Space observatory High Alps
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