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The Brightness of the Night Sky How Dark Can the Night Sky Get?

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The Brightness of the Night Sky How Dark Can the Night Sky Get? Catching the Light by Jerry Lodriguss Stories, Observations, Humor Ads by Google The Brightness of the Night Sky Introduction Saturn Sky Looking to find saturn "When darkness is at its darkest, that is the beginning of all light." sky? Visit our saturn sky - Lao-Tzu guide. DearCars.info The darkness, or brightness, of the night sky is a topic of interest to all amateur astronomers and astrophotographers. With growing light pollution we wonder how far do we have to travel to reach "dark" skies, and how much farther do we have to go to reach really dark skies? fluorescent Light How do factors such as man-made light pollution, natural air glow and light from interplanetary Clouds dust affect the darkness of the night sky? What about elevation, transparency and atmospheric Sky Scenes in particles? Fluorescent Lights Add room ambiance yourself. What color, if any, does the dark night sky have? www.fluorescentgallery.com The following material was posted to the newsgroup sci.astro.amateur in 1996 and 2001 by Brian Constellation Night Skiff, of Lowell Observatory. He has recently re-edited and updated the material and graciously Light granted permission to post it here. Twilight Turtle transforms - Jerry Lodriguss any room into a starry night sky! www.twilightturtle.com Sunlight Readable LCD How Dark Can the Night Sky Get? Transflective/High Copyright 1996 - 2001 by Brian Skiff Brightness LCD 6.4" to 42" Optical Bonding & Steve Mencinsky has suggested that I might have opinions about comparative sky brightness at Touch www.i- different sites. Indeed I do, and also can supply some facts, which have been in short supply in techcompany.com/SunLight-LCD this thread. The only person who has bothered to pursue the problem of night-sky brightness from an analytical viewpoint is Roy Garstang of the University of Colorado at Boulder. His results are most fully developed in the following publications: Dark Sky Films Featuring classic horror, Citations in the Scientific Literature oddities & "lost" film titles on DVD 1989, Publ. Astron. Soc. Pacific, 101, 306 www.darkskyfilms.com 1991, Publ. Astron. Soc. Pacific, 103, 1109 1989, Annual Reviews of Astron. & Astrophys., volume 27, page 19. Advertise on this site All three can be obtained on-line via the ADS article service at: http://adsabs.harvard.edu/article_service.html In these articles you will find the equations that allow you to compute highly accurate predictions of Ads by Google sky brightness for any site. The equations includes some "hairy integrals", but are quite amenable Stars Sky to being coded into a piece of software. The required inputs are radii to and populations of towns Dark Sky near a site of interest (and if the cities are physically large, also the radius of the city itself), and Night Sky Scope the altitude of the site. All you software jockeys should add this feature to your programs! It would Buy a Star in the Sky certainly be of interest to have values calculated for Riverside, Stellafane, Astrofest, and other star Sky Atlas party sites. I might mention that light-pollution guru Chris Luginbuhl at USNO-Flagstaff ([email protected]) has a program to do these calculations given the necessary inputs. Another first-rate source of information about the night sky in general is the marvelous little book Search by Roach and Gordon, "The Light of the Night Sky" (Reidel, 1973), which outlines the hard Can't find what you're looking science of the matter with a refreshing aesthetic attitude. for? Use Google to search Astropix.com or the entire web. And while we're at it, I may as well mention primary sources of data about the related subject of Type in your search term in the dark-adaptation (as far as I know, these are not available on-line): box below and click the appropriate button. Hulburt 1951, "Time of Dark Adaptation After Stimulation by Various Brightnesses and Colors," Journ. Optical Soc. America (JOSA), 41, 402 Smith et al. 1955, "Effects of Exposure to Various Red Lights upon Subsequent Dark Adaptation Measured by the Method of Constant Stimuli," JOSA 45, 502 Web Astropix.com Kinney 1955, "Sensitivity of the Eye to Spectral Radiation at Scotopic and Mesopic Intensity Search Levels," JOSA 45, 507 Sweeney et al. 1960, "Season Changes in Scotopic Sensitivity," JOSA 50, 237. Dark Adaption and Exposure to Bright Lights The bottom line of these papers is: red lights are best, but if you can see that it's red on the paper your looking at, it's too bright; if you go out on for long on a sunny day, expect to lose about three-quarters of a magnitude in your magnitude threshold the succeeding night---after extended exposure to high-intensity scenes (beach, snow-skiing on sunny days), it takes more than 24 hours to become fully dark-adapted! The usual half-hour or hour won't do. Wear "glacier glasses" when outside during daytime. It Can Only Get So Dark Before proceeding, it is worth noting that the s.a.a. thread so far seems to contain the implicit notion that you can find darker and darker sites if you get progressively farther from anthropogenic sources of light. This is not true, particularly from a practical standpoint. The Australian Outback, the coast northwest of Perth, the Chilean observatory sites, and isolated places in the US Southwest, plus many others have sky brightness negligibly different from the natural background, which sets a fundamental (and more-or-less inescapable) limit on how dark a site can be. The main contributions to the natural skyglow are: the zodiacal light, the night-airglow ("permanent aurora"), and scattered starlight in the atmosphere. There are also contributions from such things as scattered extragalactic "cosmic" light, but these are so small that even now their actual values are known only as upper limits. Despite the fact that many folks have not seen the zodiacal light, much less the gegenschein or zodiacal band, it is the main contribution to the natural sky brightness even the ecliptic poles. The night-airglow varies considerably due to solar activity on the time scale of minutes/hours as well as over the 11-year solar cycle, and can greatly compromise the darkness at a site on any particular night. The zodiacal light, zodiacal band, and gegenschein are prominent features of the night sky at true-dark sites. They are not tests of visual acuity, but of sky brightness. The night- airglow is also easy to see at dark sites, at least where there is little scattered light from atmospheric dust and aerosols. There are many reported visual sightings of the rippled structure in this phenomenon, looking like banded very thin altocumulus clouds. This light is visible mostly from a forbidden line of ground-state oxygen which emits at 5577A, where most light-pollution filters have their red cutoff. Gordon Garradd has some nice photos of structured airglow at his Web site: http://members.ozemail.com.au/~loomberah/agw1.htm This is the sort of thing you never see except at a "true-dark" site. The main night-sky brightness contribution---the zodiacal light---is of course just scattered sunlight, and thus is not improved by light-pollution filters. Hard Numbers on the Measure of Darkness So what's the answer? The widely accepted value for sky brightness at the zenith at a site completely free of man-made light sources and near solar activity minimum is: V mag. 22.0 per square arcsecond = mag. 13 per square arcminute. In other words, a perfect site has a sky brightness equivalent to having a mag. 22 star in every square arcsecond box (hardly bigger than the star image itself) over the entire sky. Garstang has adjusted the zero-points of his model to fit this value, which is based on actual measurements at several observatory sites around the world, and from actual experience is unexpectedly accurate. When the airglow is bright, the brightness can increase often by a full magnitude in V, but somewhat less for the dark-adapted eye, which is less sensitive to the 5577A emission than the standard photometric V filter passband. Thus even a site without light-pollution can appear "bright" to the eye depending on the state of the geomagnetic field. To escape these major effects completely you'd have to go into space well out of the plane of the solar system. Just going into Earth orbit doesn't gain you much as far as dark sky since the zodiacal light is still there. Well, what does mag. 22 per square arcsec mean for the observer? If you've been to TSP, or the Nebraska Star Party, or been on Kitt Peak or to McDonald Observatory or other similar sites on a cloud free night, then you've seen it! That's as dark as it gets. What about brighter sites, any comparisons? Here are some values from Garstang's 1989 PASP paper for some brighter sites (all the following are based on 1980 census data; adjust as you see fit): Mount Wilson 19.8 Palomar Mountain 21.5 Lick Obs. 20.7 Mount Lemmon 21.5 (near Tucson) Lowell (Mars Hill) 20.5 Van Vleck 18.7 (Connecticut) David Dunlap 18.4 (Toronto) Haute Provence 21.8 (southern France) How bright is really bright? From Flagstaff the night sky at Full Moon is about mag. 18.0. My meager experience from the centers of large cities indeed suggests the Full Moon does very little to sky brightness---rather than being the focal point of the nocturnal landscape, it's just one more street light.
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