Light Pollution in Switzerland

In Switzerland, the densely populated central plateau is most severely affected by light pollution, especially in the Aarau, Zurich and Winterthur regions. The Alpine region is very dark with the exception of a few villages in the valleys. However, on most passes in the dark areas there are hotels or restaurants that have the outside lights on all night. There is almost no way to go where it is really dark. The large skylight dome of the Italian city of Milan is visible from all over Switzerland in the south. Light pollution has increased noticeably in Switzerland over the past thirty years. Places that used to be good for stargazing have now become much too bright.

Light Pollution Maps

Light pollution map of Switzerland 2020
The map is based on the map Light Emissions of Switzerland 2020 from Dark-Sky Switzerland [498], over which a Swiss street map [499] was placed. In this way, you can easily identify the greatest light sinners in Switzerland and where you can best escape to the dark mountains to gaze at the stars. Some locations of star parties or favourite observing sites are shown.
Light pollution in Central Europe is particularly serious. There are few sparsely populated areas left with little light pollution. In Switzerland, these are the Alps from Bern to Graubünden and the Alps in Austria. These are the only areas nearby where an unobstructed view of the night sky is possible. [500]

Bortle Dark Sky Scale

This nine-level numeric scale that measures the night sky's brightness of a particular location was created by amateur astronomer John E. Bortle and published in the February 2001 edition of Sky & Telescope magazine. [501] It should help amateur astronomers evaluate and compare the darkness of an observing site.

Bortle Dark Sky Scale. BC = Bortle class, NELM = naked eye limiting magnitude (mag). SQM = sky brightness by sky quality meter (mag/arcsec2). [501, 502]
BCNELMSQMDesignation
17.6 – 8.021.99 – 22.00Excellent dark-sky site
27.1 – 7.521.89 – 21.99Typical truly dark site
36.6 – 7.021.69 – 21.89Rural sky
46.1 – 6.520.49 – 21.69Rural/suburban transition
55.6 – 6.019.50 – 20.49Suburban sky
65.1 – 5.518.94 – 19.50Bright suburban sky
74.6 – 5.018.38 – 18.94Suburban/urban transition
84.1 – 4.5< 18.38City sky
94.0Inner-city sky

Class 1: Excellent Dark-Sky Site

  • Airglow: Readily apparent, most evident within about 15° of the horizon
  • Clouds: Visible only as dark holes or voids in the starry background
  • Environment: Telescope, companions, and vehicle are almost invisible on a grass-covered field bordered by trees
  • Messier objects: Galaxy M33 is an obvious naked-eye object even with direct vision
  • Milky Way: Scorpius and Sagittarius regions cast obvious diffuse shadows on the ground
  • Sky: Jupiter or Venus in the sky seem to degrade dark adaptation
  • Visual limiting magnitude: Naked eye 7.6 to 8.0 mag, 32 cm telescope 17.5 mag, 50 cm telescope 19 mag.
  • Zodiacal light: Visible to a striking degree, band spans the entire sky, gegenschein visible

Class 2: Typical Truly Dark Site

  • Airglow: May be weakly apparent along the horizon
  • Clouds: Visible only as dark holes or voids in the starry background
  • Environment: Telescope and surroundings only vaguely visible, except where they project against the sky
  • Messier objects: M33 is rather easily seen with direct vision. Many globular clusters are distinct naked-eye objects
  • Milky Way: The summer Milky Way is highly structured to the unaided eye
  • Visual limiting magnitude: Naked eye, 7.1 to 7.5 mag, 32 cm telescope 16 or 17 mag
  • Zodiacal light: Bright enough to cast weak shadows just before dawn and after dusk. Its color can be seen as distinctly yellowish when compared with the blue-white of the Milky Way

Class 3: Rural Sky

  • Clouds: May appear faintly illuminated in the brightest parts of the sky near the horizon but are dark overhead.
  • Environment: Telescope is vaguely apparent at a distance of 6 or 9 meters.
  • Messier objects: Globular clusters such as M4, M5, M15, and M22 are all distinct naked-eye objects. M33 is easy to see with averted vision
  • Milky Way: Still appears complex
  • Sky: Some indication of light pollution is evident along the horizon
  • Visual limiting magnitude: Naked eye 6.6 to 7.0 mag, 32 cm telescope 16 mag
  • Zodiacal light: Striking in spring and autumn (when it extends 60° above the horizon after dusk and before dawn). Its color is at least weakly indicated.

Class 4: Rural/Suburban Transition

  • Clouds: In the direction of light-pollution sources are illuminated but only slightly so, and are still dark overhead.
  • Environment: Telescope rather clearly visible at a distance
  • Messier objects: M33 is a difficult averted-vision object and is detectable only when at an altitude higher than 50°
  • Milky Way: Well above the horizon is still impressive but lacks all but the most obvious structure
  • Sky: Fairly obvious domes are apparent over population centers in several directions
  • Visual limiting magnitude: Naked eye 6.1 to 6.5 mag, 32 cm telescope 15.5 mag
  • Zodiacal light: Clearly evident but doesn't even extend halfway to the zenith at the beginning or end of twilight

Class 5: Suburban Sky

  • Clouds: Over most or all of the sky, clouds are quite noticeably brighter than the sky itself
  • Milky Way: is very weak or invisible near the horizon and looks rather washed out overhead
  • Sky: Light sources are evident in most if not all directions
  • Visual limiting magnitude: Naked eye 5.6 to 6.0 mag, 32 cm telescope 14.5 to 15 mag
  • Zodiacal light: Only hints are seen on the best spring and autumn nights

Class 6: Bright Suburban Sky

  • Clouds: Anywhere in the sky appear fairly bright
  • Environment: No trouble seeing eyepieces and telescope accessories on an observing table
  • Messier objects: M33 is impossible to see without binoculars. M31 is only modestly apparent to the unaided eye
  • Milky Way: Any indications are apparent only toward the zenith
  • Sky: The sky within 35° of the horizon glows grayish white
  • Visual limiting magnitude: Naked eye about 5.5 mag, 32 cm telescope 14.0 to 14.5 mag
  • Zodiacal light: No trace can be seen, even on the best nights

Class 7: Suburban/Urban Transition

  • Clouds: Are brilliantly lit
  • Messier objects: M44 or M31 may be glimpsed with the unaided eye but are very indistinct. The brightest Messier objects are pale ghosts of their true selves
  • Milky Way: Totally invisible or nearly so
  • Sky: The entire sky background has a vague, grayish white hue. Strong light sources are evident in all directions
  • Visual limiting magnitude: Naked eye 5.0 if you really try, 32 cm telescope barely reach 14th mag

Class 8: City Sky

  • Messier objects: M31 and M44 may be barely glimpsed by an experienced observer on good nights
  • Sky: The sky glows whitish gray or orangish, and you can read newspaper headlines without difficulty. Some of the stars making up the familiar constellation patterns are difficult to see or are absent entirely
  • Visual limiting magnitude: Naked eye down to magnitude 4.5 at best, 32-cm telescope little better than 13 mag

Class 9: Inner-City Sky

  • Messier objects: Aside from perhaps the Pleiades, no Messier objects are visible to the unaided eye
  • Sky: The entire sky is brightly lit, even at the zenith. Many stars making up familiar constellation figures are invisible, and dim constellations such as Cancer and Pisces are not seen at all.
  • Visual limiting magnitude: Naked-eye 4.0 or less.

Links on the Subject of Light Pollution

References

498Dark-Sky Switzerland Nachtkarten; darksky.ch/dss/de/aktivitaeten/nachtkarten (2022-02-26)
499Kostenlose Schweizerkarten mit Kantone und Seen zum Ausdrucken; vorla.ch/schweizerkarte (2022-02-26)
500Light Pollution Map; lightpollutionmap.info (2022-02-27)
501«Gauging Light Pollution: The Bortle Dark-Sky Scale» by John E. Bortle, Sky & Telescope, February 2001; skyandtelescope.org/astronomy-resources/light-pollution-and-astronomy-the-bortle-dark-sky-scale (2022-02-27)
502Dark Skies Awareness, Sky Brightness Nomogram; darkskiesawareness.org/nomogram.php (2022-02-27)