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  • blackstar7us
    Rate Your Skies 1-2-3-4-5-6-7-8-9. Class 1: Excellent dark-sky site. The zodiacal light, gegenschein, and zodiacal band (S &T: October 2000, page 116) are all
    Message 1 of 1 , Nov 4, 2002
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      Rate Your Skies 1-2-3-4-5-6-7-8-9.

      Class 1: Excellent dark-sky site. The zodiacal light, gegenschein,
      and zodiacal band (S &T: October 2000, page 116) are all visible -
      the zodiacal light to a striking degree, and the zodiacal band
      spanning the entire sky. Even with direct vision, the galaxy M33 is
      an obvious naked-eye object. The Scorpius and Sagittarius region of
      the Milky Way casts obvious diffuse shadows on the ground. To the
      unaided eye the limiting magnitude is 7.6 to 8.0 (with effort); the
      presence of Jupiter or Venus in the sky seems to degrade dark
      adaptation. Airglow (a very faint, naturally occurring glow most
      evident within about 15 degrees of the horizon) is readily apparent.
      With a 32-centimeter (12½) scope, stars to magnitude 17.5 can be
      detected with effort, while a 50-cm (20-inch) instrument used with
      moderate magnification will reach 19th magnitude. If you are
      observing on a grass-covered field bordered by trees, your telescope,
      companions, and vehicle are almost totally invisible. This is an
      observer's Nirvana!

      Class 2: Typical truly dark site. Airglow may be weakly apparent
      along the horizon. M33 is rather easily seen with direct vision. The
      summer Milky Way is highly structured to the unaided eye, and its
      brightest parts look like veined marble when viewed with ordinary
      binoculars. The zodiacal light is still bright enough to cast weak
      shadows just before dawn and after dusk, and its color can be seen as
      distinctly yellowish when compared with the blue-white of the Milky
      Way. Any clouds in the sky are visible only as dark holes or voids in
      the starry background. You can see your telescope and surroundings
      only vaguely, except where they project against the sky. Many of the
      Messier globular clusters are distinct naked-eye objects. The
      limiting naked-eye magnitude is as faint as 7.1 to 7.5, while a 32-cm
      telescope reaches to magnitude 16 or 17.

      Class 3: Rural sky. Some indication of light pollution is evident
      along the horizon. Clouds may appear faintly illuminated in the
      brightest parts of the sky near the horizon but are dark overhead.
      The Milky Way still appears complex, and globular clusters such as
      M4, M5, M15, and M22 are all distinct naked-eye objects. M33 is easy
      to see with averted vision. The zodiacal light is striking in spring
      and autumn (when it extends 60 degrees above the horizon after dusk
      and before dawn) and its color is at least weakly indicated. Your
      telescope is vaguely apparent at a distance of 20 or 30 feet. The
      naked-eye limiting magnitude is 6.6 to 7.0, and a 32-cm reflector
      will reach to 16th magnitude.

      Class 4: Rural/suburban transition. Fairly obvious light-pollution
      domes are apparent over population centers in several directions. The
      zodiacal light is clearly evident but doesn't even extend halfway to
      the zenith at the beginning or end of twilight. The Milky Way well
      above the horizon is still impressive but lacks all but the most
      obvious structure. M33 is a difficult averted-vision object and is
      detectable only when at an altitude higher than 50 degrees. Clouds in
      the direction of light-pollution sources are illuminated but only
      slightly so, and are still dark overhead. You can make out your
      telescope rather clearly at a distance. The maximum naked-eye
      limiting magnitude is 6.1 to 6.5, and a 32-cm reflector used with
      moderate magnification will reveal stars of magnitude 15.5.
      Class 5: Suburban sky. Only hints of the zodiacal light are seen on
      the best spring and autumn nights. The Milky Way is very weak or
      invisible near the horizon and looks rather washed out overhead.
      Light sources are evident in most if not all directions. Over most or
      all of the sky, clouds are quite noticeably brighter than the sky
      itself. The naked-eye limit is around 5.6 to 6.0, and a 32-cm
      reflector will reach about magnitude 14.5 to 15.

      Class 6: Bright suburban sky. No trace of the zodiacal light can be
      seen, even on the best nights. Any indications of the Milky Way are
      apparent only toward the zenith. The sky within 35 degrees of the
      horizon glows grayish white. Clouds anywhere in the sky appear fairly
      bright. You have no trouble seeing eyepieces and telescope
      accessories on an observing table. M33 is impossible to see without
      binoculars, and M31 is only modestly apparent to the unaided eye. The
      naked-eye limit is about 5.5, and a 32-cm telescope used at moderate
      powers will show stars at magnitude 14.0 to 14.5.

      Class 7: Suburban/urban transition. The entire sky background has a
      vague, grayish white hue. Strong light sources are evident in all
      directions. The Milky Way is totally invisible or nearly so. M44 or
      M31 may be glimpsed with the unaided eye but are very indistinct.
      Clouds are brilliantly lit. Even in moderate-size telescopes, the
      brightest Messier objects are pale ghosts of their true selves. The
      naked-eye limiting magnitude is 5.0 if you really try, and a 32-cm
      reflector will barely reach 14th magnitude.
      Class 8: City sky. The sky glows whitish gray or orangish, and you
      can read newspaper headlines without difficulty. M31 and M44 may be
      barely glimpsed by an experienced observer on good nights, and only
      the bright Messier objects are detectable with a modest-size
      telescope. Some of the stars making up the familiar constellation
      patterns are difficult to see or are absent entirely. The naked eye
      can pick out stars down to magnitude 4.5 at best, if you know just
      where to look, and the stellar limit for a 32-cm reflector is little
      better than magnitude 13.

      Class 9: Inner-city 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. Aside from perhaps the Pleiades, no Messier objects are
      visible to the unaided eye. The only celestial objects that really
      provide pleasing telescopic views are the Moon, the planets, and a
      few of the brightest star clusters (if you can find them). The naked-
      eye limiting magnitude is 4.0 or less.
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