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(meteorobs) THE BORTLE DARK SKY SCALE
Print out the following and take it with you into the field!
Lew Gramer
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THE BORTLE DARK SKY SCALE
by John Bortle
Forwarded with permission, for personal use only. Not to be reproduced
or used for any other reason, commercial or public.
Class 1: Excellent dark-sky site. The zodiacal light, gegenschein,
and zodiacal band 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 12" telescope stars to
magnitude 17.5 can be detected with effort, while a 20" 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 naked 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 to 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 12"
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 12" reflector
will reach to 16th magnitude.
Class 4: Rural / suburban transition. Fairly obvious light pollution
domes are obvious over population centers in several directions. The
zodiacal light is clearly evident, but doesn't extend even
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 at an altitude of 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 12" 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 12"
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 12" 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 12"
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. M44 and M31 may be
barely glimpsed by an experience 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 12" 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 like 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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