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Bortle Dark Sky Classification Essay

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The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the night sky 's and stars ' brightness (naked-eye and stellar limiting magnitude ) of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution and skyglow. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers compare the darkness of observing sites. The scale ranges from class 1, the darkest skies available on Earth, through class 9, inner-city skies. [ 1 ]

The table below [ 2 ] summarizes Bortle's descriptions of the classes. The colors are from the World Atlas of Artificial Night Sky Brightness. [ 3 ] and are provided as a convenience to the reader. The correlation between the colors and Bortle classes is approximate at best.

Only hints of zodiacal light are seen on the best nights in autumn and spring; Milky Way is very weak or invisible near the horizon and looks washed out overhead; light sources visible in most, if not all, directions; clouds are noticeably brighter than the sky.

Bright suburban sky

Zodiacal light is invisible; Milky Way only visible near the zenith; sky within 35° from the horizon glows grayish white; clouds anywhere in the sky appear fairly bright; surroundings easily visible; M33 is impossible to see without at least binoculars. M31 is modestly apparent to the unaided eye.

Entire sky has a grayish-white hue; strong light sources evident in all directions; Milky Way invisible; M31 and M44 may be glimpsed with the naked eye, but are very indistinct; clouds are brightly lit; even in moderate-sized telescopes the brightest Messier objects are only ghosts of their true selves.

Sky glows white or orange—one can easily read; M31 and M44 are barely glimpsed by an experienced observer on good nights; even with telescope, only bright Messier objects can be detected; stars forming familiar constellation patterns may be weak or completely invisible.

Sky is brilliantly lit with many stars forming constellations invisible and many weaker constellations invisible; aside from Pleiades. no Messier object is visible to the naked eye; only objects to provide fairly pleasant views are the Moon. the Planets. and a few of the brightest star clusters .

See also References

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John Bortle's Dark Sky Scale

John Bortle created a new scale of sky darkness which he published in the February 2001 issue of Sky and Telescope. Doug Stewart requested and received John Bortle's kind permission to repost his copy righted Dark Sky Scale here. As John Bortle makes clear in his article, using limiting magnitude's as a measure of sky darkness is totally subjective and idiosyncratic. I've always used the Pleiades as a guide. Count them and you'll easily see five stars. I can usually see Celaeno and Asterope (seven stars). I would estimate the sky at 5.5 to 6.0, but kids can often see 22 Tauri at magnitude 6.4. As John Bortle also points out, diffuse objects like nebulae and galaxies are harder to see the bright point sources of light. Color makes a difference as well. I have only seen Uranus (magnitude about 5.8) with my eyes alone once or twice. Its soft green color is too easy for me to lose in the sky.

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° of the horizon) is readily apparent. With a 12½" scope 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 12½" 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° 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 12½" 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°. 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 12½" 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 12½" 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° 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 12½" 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 12½" 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 12½" 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.

This document was authored by Les Coleman and is subject to Copyrights belonging to Les Coleman. This material may be referenced and reproduced as long as proper attribution is given as specified in Proper Usage Guidelines for Frosty Drew and Related Materials .

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Bortle dark sky scale

Bortle Dark Sky Scale

 
The Bortle Dark Sky Scale was developed by John Bortle "based on nearly 50 years of observing experience," to assess the amount of light pollution in a night sky. It was first published in a 2001 Sky & Telescope article.

To facilitate learning and using the scale, I've adapted Bortle's indicators of sky brightness as a table (below), including the color codes used in available light pollution maps .

For the amateur astronomer, the most robust and convenient relative measure of sky brightness is the naked eye or telescopic limiting magnitude. This is also a criterion that can be directly reported without recourse to the Bortle classification categories.

The five star charts below document stars down to visual magnitude 7.7 in sky areas 25° on a side that culminate near the zenith for continental United States observers (δ  = 18° to 43°, with the exception of Lyra-Hercules and Equuleus-Deliphinus). Although these areas are not evenly spaced around the celestial sphere (to avoid the effect of Milky Way background brightness), at least one should be convenient to observe near the zenith at any time of the year.

To calculate the sky darkness using these charts, simply canvas the entire area of the chart and mark as many stars as you can recognize that are near your averted vision threshold. Do not mark stars that you can identify with direct vision or that are easy with averted vision; try to select stars near your threshold. Identify in this way at least 10 faint stars. Later, tally the number of stars that fall within each magnitude bin shown in the key at bottom left, which identifies the half magnitude steps corresponding to the Bortle categories. The prevailing sky brightness is the average magnitude of the two faintest bins marked:

where t is a tally and m is the fainter bracket magnitude that defines the magnitude interval bin. For example, at my home location I tallied 7 stars of magnitude 5.0–5.49 and 9 stars of magnitude 5.5–5.99, so:

SB = (7*5.5+9*6.0)/(7+9) = (38.5+54)/16 = 5.78 = Bortle 5 (suburban)

Your limit magnitude may differ from another observer's, but this difference in visual acuity will transfer to all other visual tasks. The Bortle scale inevitably combines differences in sky brightness and differences in individual detection capabilities.

Bortle Dark-Sky Scale

The Bortle Dark-Sky Scale

Use the chart below with the Light Pollution maps to guage the quality of your night skies where you live.

The column labeled "Naked-eye Limiting Magnitude" indicates the dimmest stars visible under each class of light pollution. The larger the magnitude number is, the dimmer the star is. Each whole number represents a factor of 5 in brightness. In other words, a magnitude 5 star appears five times brighter than a magnitude 6 star, while a magnitude 4 star appears ten times brighter than a magnitude 6 star.

Naked-eye Limiting Magnitude

Zodiacal Light / Constellations

Airglow and Clouds

Night Time Scene

Excellent, truly dark-skies.

MW shows great detail and light from the Scorpio / Sagittarius region casts obvious shadows on the ground.

M33 (the Pinwheel Galaxy) is a obvious object.

Zodiacal light has an obvious color and can stretch across the entire sky.

Bluish airglow is visible near the horizon and clouds appear as dark blobs againt the backdrop of the stars.

The brightness of Jupiter and Venus is annoying to night vision. Ground objects are barely lit and trees and hills are dark.

Typical, truly dark skies.

Summer MW shouws great detail and has veined appearance.

M33 is visible with direct vision, as are many globular clusters.

Zodiacal light bright enough to cast weak shadows after dusk and has an apparent color.

Airglow may be weakly apparent and clouds still appear as dark blobs.

Ground is mostly dark, but objects projecting into the sky are discernible.

Dark Skies, Bright Stars

Dark Skies, Bright Stars

The Milky Way as seen from Cherry Springs

Just North of State College is a region of PA known as the “Pennsylvania wilds” This area is mountainous and sparse, there are very few towns and huge expanses of wilderness. Whitetail Deer, Black Bear, Wild Turkey and Elk are abundant due to comparatively low hunting pressure. It is an incredibly special place because of the fact that it is so underused. Unlike Parks like Acadia, Great Smoky, or Yellowstone, north central PA does not attract many tourists, and for the most part the forests are deep, dark, and silent.

One area of the PA wilds known as the black forest is so far from civilization that it has almost no light pollution. Astronomers and amateur stargazers alike have been flocking to this remote part of Potter County since the mid 90s to experience the last dark place on the east coast. In Cherry Springs State Park, a 40 acre observation field was cleared in order to make room for stargazers. On a clear night, 50-100 people show up to see the stars and during meteorological events like meteor showers, hundreds upon hundreds of enthusiasts gather for “star parties” in the field.

Dark Sky Map of PA, Cherry Springs lies in the heart of the black dot in Potter County

Cherry Springs is classified as a “true dark sky”, a level 2 on the Bortle Dark-Sky scale. This is remarkable because dark skies are so rare in this part of the country due to the high population density of the eastern seaboard. As urban sprawl continues to spread our skies are becoming more and more polluted. 99% of the population of Europe and North America live in an area where they cannot see unpolluted starry skies at night. I feel as though seeing the stars at Cherry Springs is a way to escape to a time and place when man did not have as much of an impact on the earth. Dark skies also seem to be something incredibly fragile, something that many not be so readily viewable a few years down the road.

Typically there are around 60 clear nights a year at Cherry Springs, and very few of them occur in the winter. Cherry springs is exceptionally cold, snowy, and foggy in the winter. However, last Saturday night was cold, crisp and clear so my girlfriend and I decided to drive to see the stars at Cherry Springs.

We hiked on the Susquehannock trail to the Cherry Springs fire tower which provided an awesome view of the surrounding forest. As the sun went down the temperature began to drop. It was more than 20 degrees colder in the PA wilds than it was in State College so we decided to hike back to the car and sleep there rather than pitching a tent in the deep snow. We cooked dinner on the tailgate and watched as the stars came out. Even before the twilight had faded the sky was filled with hundreds of brilliant stars.

Cherry Springs is one of the few places in the Continental US one can go to see the Northern Lights

We fell asleep for a while and woke up at 2 am to see the stars. It was only about 5 degrees out and the car had frozen up so we put on all of our warmest clothing and sat up on the roof. What we saw amazed us. The sky was completely lit up by thousands of points of light. Easily identifiable constellations like the Big Dipper and Orion’s Belt were hard to see on account of the thousands of stars scattered across the sky like salt spilled on a black table cloth. Probably most impressive was the Milky Way. The Milky Way was an easily visible cluster of stars stretching across the night sky. The stars were so numerous that they blended together in places to form a continuous cluster of light.

Nowhere in my travels have I seen stars like the ones at Cherry Springs, the Black Forest of Pa is truly a special place and the night sky there is other worldly. Staying in Cherry Springs for a night of stargazing is a trip that all Penn Staters should experience.

Post navigation 3 thoughts on “ Dark Skies, Bright Stars ”

I know what you mean when you say the Stars are amazing. When I was backpacking in New Mexico the Stars were probably the most amazing part of the trip. Back home, I am only ever able to see the big dipper (maybe). I will say that if you walk up to the arboretum here at Penn State at night the stars are sometimes very impressive.

Seeing the night sky without light pollution is pretty incredible. I was in the Sonoran Desert this winter camping and it was absolutely phenomenal to see the stars that are normally hidden by all of the extra light in areas like State College.

The picture you posted at the beginning is pretty incredible. I don’t know if it’s one that you took or if you pulled it off the internet, but it looks like something that belongs in National Geographic. I had no idea you could see the stars like that anywhere near here. It actually prompted me to look up if there were any “Dark Sky” areas near my hometown. It turns out there’s nothing even close in Connecticut. It looks like I should probably take advantage of Cherry Springs while I have the chance.

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Bortle 1 Skies

Bortle 1 Skies

Without actually seeing just what you are seeing it is a bit difficult to precisely judge the problem. But, overall I honestly don't take a lot of stock in today's various light pollution maps, at least beyond as a rough approximation. At least one of the maps even suggests that my local conditions are growing steadily better over time when, in fact, they are slowly deteriorating!

Under a true Bortle Class 1 sky the conditions are indeed so striking in the number of stars visible to the unaided eye that for most of us the general constellation outlines are all but lost against the background density of fainter stars. Likewise, individual faint stars to magnitude 7.5. or better, can be detected by those with 20/20 vision and DSO like M33 are clearly naked eye objects. This is a fact that I can fully attest to, since I lived under such skies for more than a decade.

Definitely Air Glow is not the culprit to what you are reporting as a general sky glow and the solar cycle is on the down-turn anyway and Air Glow pretty much is situated within

15 degrees of the horizon.

I do note that a number of West Coast observers are reporting that jet contrails are increasingly becoming very troublesome, the individual trails diffusing to merge into a vast sheet of very thin cirrus-like structure over the entire sky. If this is drifting eastward toward AZ and NM and finding even very widely separated light pollution sources it might be enough to account for what you report.

The above is about the best suggesting that I can offer, at least beyond the simple spread of universally growing light pollution (urban sprawl) as the U.S. population steadily increases, particularly in the American Southwest.

The Space Review: Review: The End of Night

Review: The End of Night by Jeff Foust
Monday, August 26, 2013

Some of the most popular pictures of the Earth from space are taken at night. The natural features of our planet are largely invisible in those images, but the artificial ones—the lights of our cities, primarily—stand out. The patterns of those cities become evident in those pictures, and in some cases the geography is literally illuminated as well. as cities hug coastlines and follow rivers (see “Review: Lights of Mankind”. The Space Review, December 12, 2011). You don’t have to be an astronaut on the International Space Station to get that same experience: flying in an airliner on a clear night also offers a similar perspective, albeit from an altitude of a dozen kilometers versus 400 kilometers, the cities passing below becoming a form of live geography quiz.

Instead of going from one to nine, the book’s chapters run from nine to one. Bogard makes use of what’s known as the Bortle Dark-Sky Scale, a one-to-nine classification of the darkness of the night sky.

These artificial constellations may look pretty, but they have a serious downside as well: the fact that these lights are visible from space means that the people living in those cities are deprived of a view of the natural constellations of the night sky. Light pollution is becoming a more serious issue as cities get more, and brighter, lights, attempting, in essence, to abolish the night. The implications of light pollution extend beyond simply the inability to see many stars in the night sky to potentially significant psychological, physiological, and even philosophical issues, as explored by Paul Bogard in The End of Night.

One of the first things you’ll notice about Bogard’s book is the enumeration of the chapters: instead of going from one to nine, they run from nine to one. Bogard makes use of what’s known as the Bortle Dark-Sky Scale, a one-to-nine classification of the darkness of the night sky developed by astronomer John Bortle. A one is a very dark site (“an observer’s Nirvana!” Bortle wrote in a 2001 Sky & Telescope article ) where even the Milky Way can cast shadows, while a nine is a bright inner city sky where only the brightest of stars can be seen. In The End of Night. Bogard starts in those bright city lights—the Las Vegas Strip—and makes it to some of the few remaining very dark sites in North America in the final chapter.

Along the way, Bogard travels across North America and Europe, looking not just at the visibility of the night sky but the history of artificial lighting of cities and the consequences of such lighting. More and brighter lighting is often defended on public safety grounds—by banishing darkness, proponents argue, they give criminals no place to hide—but there’s little evidence, he notes, of a correlation between increased lighting and decreased crime. People who work late shifts, exposed only to bright artificial lights, run increased risks of everything from obesity to, more controversially, cancer. And there is also the loss of connection to the night sky, with most people now living in cities and suburbs whose skies are so bright that the Milky Way is never visible.

If we are ever going to protect the darkest places we have left, they will have to be dark places we actually know and visit, love and respect,” he writes.

The End of Night is not without some hope, though. Bogard points to efforts or groups like the International Dark-Sky Association (IDA), which supports efforts like local ordinances to reduce light pollution in cities. Even for IDA success stories like Flagstaff, Arizona, which has strong measures in place to reduce light pollution, they have only reduced the rate of growth of overall illumination as the city grows overall. IDA also recognizes locations with particularly dark skies that make efforts to preserve them (one, Mont-Mégantic National Park in Quebec, calls itself in French “Réserve internationale de ciel étoile” or “International starry sky reserve,” which perhaps sounds more positive than the IDA’s designation of “dark sky reserve.”)

Near the end of the book, Bogard visits some of the few remaining sites with “Bortle class one” skies. The viewing (when not obscured by clouds or smoke from wildfires) is stunning, but these locations are difficult to get to—if they were easy, their skies would likely be not as dark. An alternative, he suggests, could be those national parks in parts of the country, particularly the Southwest, where skies are nearly as dark but are sites that are more accessible to the public. That fits into the overall mission of the National Park Service, he argues, of preserving nature for future generations. “If we are ever going to protect the darkest places we have left, they will have to be dark places we actually know and visit, love and respect,” he writes.

The End of Night serves as a complement to another recent work on light pollution, the 2011 documentary The City Dark (see “Review: The City Dark”. The Space Review, February 6, 2012.) The documentary can offer the visuals of the effects of light pollution on the night sky not possible in a book, while this book goes into a level of detail not possible in a 90-minute documentary. Both, though, offer a warning that society’s desire for more and brighter lights is not without astronomical and other consequences.

Jeff Foust (jeff@thespacereview.com ) is the editor and publisher of The Space Review. He also operates the Spacetoday.net web site and the Space Politics and NewSpace Journal weblogs. Views and opinions expressed in this article are those of the author alone, and do not represent the official positions of any organization or company, including the Futron Corporation, the author’s employer.

Bortle - s Dark-Sky Scale

Bortle’s Dark-Sky Scale

Ever since the invention of fire, humans have sought to banish the night. Being creatures that don’t see very well after the Sun has set, we have been on a quest to turn night into day for thousands of years. With the advent of the modern electrical light, not only is this a possibility – but a reality. There are hundreds, if not thousands of cities that have completely eliminated the night, cities such as New York and Beijing, are so bright that night has been eliminated. Even smaller cities cast enough light to eliminate our handicap, allowing us to function in the dark. But this has come at a cost – light pollution .

As our electric lights brighten our night, they also brighten our sky. The light emitted from these light sources reflects off our own atmosphere and brighten the sky, causing a phenomenon called Skyglow. Because this brightens our sky, it washes out the dimmer stars in the heavens. The more light pollution you have, the more stars get washed out.

John Bortle came up with a scale to measure the darkness of the sky, this scale is called the Bortle Dark-Sky Scale and it was first published in Sky & Telescope magazine. It was published to help amateur astronomers quantify the amount of light pollution present in their observation sites, giving them a scale to work with in their quest to see the cosmos. The scale is a 1-9 numbered scale, with a Class 1 sky having zero light pollution and Class 9 having the most.

To summarize the classes of the Bortle D ark-Sky Scale

There are several organizations that seek to limit light pollution and protect the darkest areas on Earth in the same way national parks protect the land. Personally, I live in Portland, Oregon and I’d say the sky here is a Class 7, the darkest sky I’ve seen is definitely a Class 2 (maybe even a Class 1, out here we have some of the last class 1 skies left in America).

What about you? How would you describe the sky in your hometown and what is the darkest sky you’ve ever seen?

For more information about combating light pollution, visit the International Dark-Sky Association ‘s website.