มาตราบอร์เทิล
ตารางการจำแนกประเภทท้องฟ้ามืด

| คลาส | ชื่อ | NELM | ประมาณตารางเมตร โชติมาตร/พิลิปดา2 | คำอธิบาย |
| --- | --- | --- | --- | --- |
| 1 | Excellent dark-sky site | 7.6–8.0 | 21.76 - 22.0 |  แสงจักรราศีมองเห็นได้และมีสีสัน  แถวแสงรูปวงรีมองเห็นได้อย่างชัดเจน  แถบแสงจักรราศีมองเห็นได้  แสงเรืองบนท้องฟ้ามองเห็นได้ชัดเจน  บริเวณกลุ่มดาวแมงป่องและกลุ่มดาวคนยิงธนู ของทางเชือกเผือก ทำให้เกิดเงาที่มองเห็นได้ชัดเจน  หลายกลุ่มดาว โดยเฉพาะกลุ่มดาวที่มองเห็นได้ยาก มองเห็นได้ยากขึ้นท่ามกลางดวงดาวที่มากมาย  วัตถุท้องฟ้าของเมซีเยและกระจุกดาวทรงกลมหลายแห่งสามารถมองเห็นได้ด้วยตาเปล่า  M33 (ดาราจักรสามเหลี่ยม) มองเห็นได้ด้วยตาเปล่าโดยตรง  ความสามารถในการมองเห็นดาวที่มีโชติมาตรปรากฏต่ำสุด (limiting magnitude) เมื่อใช้กล้องโทรทรรศน์สะท้อนแสงขนาด 32 เซนติเมตร (12.5 นิ้ว) อยู่ที่ 17.5 (โดยต้องใช้ความพยายาม)  ดาวศุกร์และดาวพฤหัสบดี มีผลต่อการปรับการมองเห็นในความมืด |
| 2 | Typical truly dark site | 7.1–7.5 | 21.6–21.75 |  the zodiacal light is distinctly yellowish and bright enough to cast shadows at dusk and dawn  airglow may be weakly visible near horizon  the gegenschein is visible  clouds are only visible as dark holes against the sky  surroundings are barely visible silhouetted against the sky  the summer Milky Way is highly structured  many Messier objects and globular clusters are naked-eye objects  M33 is easily seen with naked eye  limiting magnitude with 12.5" reflector is 16.5 |
| 3 | Rural sky | 6.6–7.0 | 21.3–21.6 |  the zodiacal light is striking in spring and autumn, and color is still visible  some light pollution evident at the horizon  clouds are illuminated near the horizon, dark overhead  nearer surroundings are vaguely visible  the summer Milky Way still appears complex  M15, M4, M5, and M22 are naked-eye objects  M33 is easily visible with averted vision  limiting magnitude with 12.5" reflector is 16 |
| 4 | Brighter rural | 6.3–6.5 | 20.8–21.3 |  the zodiacal light is still visible, but does not extend halfway to the zenith at dusk or dawn  light pollution domes visible in several directions  clouds are illuminated in the directions of the light sources, dark overhead  surroundings are clearly visible, even at a distance  the Milky Way well above the horizon is still impressive, but lacks detail  M33 is a difficult averted vision object, only visible when high in the sky  limiting magnitude with 12.5" reflector is 15.5 |
| 4.5 | Semi-Suburban/Transition sky | 6.1–6.3 | 20.3–20.8 |  clouds have a grayish glow at zenith and appear bright in the direction of one or more prominent city light domes  the Milky Way is only vaguely visible – 10–15 degrees above the horizon. However the Great Rift, when overhead and with good transparency, is still obvious.  Although the views of bright globular clusters through 10" aperture and larger are striking, the outer regions of galaxies are difficult or impossible to see.  limiting magnitude with 12.5" reflector is 15.2 |
| 5 | Suburban sky | 5.6–6.0 | 19.25–20.3 |  only hints of zodiacal light are seen on the best nights in autumn and spring  light pollution is visible in most, if not all, directions  clouds are noticeably brighter than the sky  the Milky Way is invisible near the horizon, and looks washed out overhead. The winter Milky Way, even directly overhead, is fairly subtle.  when it is half moon (first/last quarter) in a dark location the sky appears like this, but with the difference that the sky appears dark blue  limiting magnitude with 12.5" reflector is 15 |
| 6 | Bright suburban sky | 5.1–5.5 | 18.5–19.25 |  the zodiacal light is invisible  light pollution makes the sky within 35° of the horizon glow grayish white  clouds anywhere in the sky appear fairly bright  even high clouds (cirrus) appear brighter than the sky background  surroundings are easily visible  the Milky Way is only visible near the zenith  M33 is not visible, M31 is modestly apparent  limiting magnitude with 12.5" reflector is 14.5 |
| 7 | Suburban/urban transition | 4.6–5.0 | 18.00–18.5 |  light pollution makes the entire sky light gray  strong light sources are evident in all directions  clouds are brightly lit  the Milky Way is nearly or totally invisible  M31 and M44 may be glimpsed, but with no detail  through a telescope, the brightest Messier objects are pale ghosts of their true selves  when it is full moon in a dark location the sky appears like this, but with the difference that the sky appears blue  limiting magnitude with 12.5" reflector is 14 |
| 8 | City sky | 4.1–4.5 | <18.00 |  the sky is light gray or orange – one can easily read  stars forming familiar constellation patterns may be weak or invisible  M31 and M44 are barely glimpsed by an experienced observer on good nights  even with a telescope, only bright Messier objects can be detected  limiting magnitude with 12.5" reflector is 13 |
| 9 | Inner-city sky | 4.0 | <18.00 |  The sky is brilliantly lit  many stars forming constellations are invisible and many fainter constellations are invisible  aside from the Pleiades, no Messier object is visible to the naked eye  the only objects to observe are the Moon, the planets, bright satellites, and a few of the brightest star clusters |