sign in sign up
<<
Home , Bortle scale, Limiting magnitude, Naked eye, Night sky, Sky brightness

Chadwick A. Moore Visual Estimations of Brightness

We can no longer avoid the issue of , because soon we will have nowhere left to go. The global problem of light pollution has never been so artfully expressed as in Woodruff T. Sullivan’s famous ‘ at Night’ images.* Kosai and Isobe 1992

anagers of parks, preserves, refuges, and wilderness areas are becoming increasingly concerned over the loss in visibility of the . Like the apocalyptic threat of Rachel Carson’s Silent Spring, those who cherish the night fear a night of no , with only the sallow glow of streetlights for inspira- Mtion. Encroaching city lights scatter light upward, bathing the otherwise dark sky and reducing the contrast to a point where stars are lost in urban glow. The effect of urban lighting, also known as light pollution, can reach surpris- ingly far. For example, from Death Valley National Park the lights of Las Ve- gas produce an obvious and obtrusive glow even though the city is 100 miles to the southeast. Los Angeles, 160 miles to the southwest, produces a dim but broad glow across the southern (Moore and Duriscoe, in prep.). Astronomers at observatories contrast, and atmospheric . were perhaps first to notice this However, less costly tools are avail- problem. As early as 1970, astrono- able to those interested in monitoring mers were scouring the USA for suit- their dark sky resource. The simplest able observing sites away from city and least costly monitoring methods lights; remaining opportunities were are visual estimations using the hu- few (Walker 1970; Garstang 1989). man eye. Today, astronomical observatories Under a pristine dark sky, per- employ multi-million-dollar equip- haps 14,000 stars in the celestial ment to measure , sphere would be discretely visible ______*

46 The George Wright FORUM (Figure 1). Outdoor lighting tends to trast between the background and scatter light upward, brightening the fainter stars until they become invisi- background of space. This increase ble to the eye. Also lost with the stars in sky brightness reduces the con- are the diffuse objects in the sky—

Volume 18 • Number 4 2001 47 nebulae, , , and the ever, what the eye lacks in precision, river of stars in our called the it makes up for in sensitivity and ease . The visible loss of these of use. The scoptic (grayscale) vision faint and diffuse astronomical objects we use at night is surprisingly sensi- is what troubles amateur astronomers tive, able to detect as few as 200 so much. This group has been the photons per second falling on the most vocal in opposing light pollu- retina and transmitting a message to tion and promoting the conservation the brain (Russell 1917). The eye’s of dark night skies. rod cells are 1,000 times more sensi- Amateur astronomers and meteor tive than the color-detecting cone observers have made visual estima- cells (Carr et al. 1989b). Scoptic vi- tions of limiting for years. sion is most sensitive in the greens is a measure of and blues, and least sensitive to the the brightness of the faintest one reds; thus the use of red-filtered can see. The astronomical magnitude flashlights to preserve night vision. scale increases with faintness. Mag- The star magnitudes used in this nitude zero represents bright stars method are measured in the “John- such as , , or , son V” spectrum, which closely while magnitude 7 stars would be matches the human eye’s scoptic vi- near the faint limit of most dark skies. sion, and are therefore an appropri- In exceptional cases, magnitude 8 ate analogue for brightness meas- stars have been observed with the urements. (Russell 1917; Bowen 1947). The number of visible stars, and the diversity of astronomical The visual estimation of limiting objects visible, decrease rapidly as magnitude is based on star counts of the limiting magnitude falls. It is not 25 established sample areas (similar uncommon for a remote area sur- to methods utilized by meteor ob- rounded by rapid urbanization to servers; Figure 2). Each area contains lose more than half the visible stars in a field of mapped stars with known a decade (Moore and Duriscoe, in brightness values. The observer prep.). scans the field using averted vision, The human eye is a somewhat trying to detect sequentially fainter imprecise instrument. Few people stars on the map. The faintest star have 20/20 vision without aid of observed becomes the sky’s limiting lenses, and the eye changes in light- magnitude (LM). By following the gathering capability and acuity with procedures for dark adaptation and age (Carr et al. 1989b). There is also counting, reasonable conformity can potential for bias in the eye’s central be attained between observers processing unit— the brain. How- (Blackwell 1946).

48 The George Wright FORUM Initially, this star count is con- fying some light sources while sup- ducted at the zenith (straight over- pressing others. The effect of local head). Counts can also be conducted weather upon sky brightness is an in quadrants of the sky, and at vari- interesting study in itself, but such ous angular altitudes above the hori- conditions should be avoided to zon. The process can take as little as produce a baseline inventory to track 30 minutes to arrive at a zenith LM long-term changes. number. Observers have used this methodology to produce brightness maps of different parts of the night sky, or to take single measurements Light scattered upward is not the on multiple to capture the only factor affecting an LM meas- range of variation associated with urement. Pollutants in the atmos- weather, seasonal changes, or atmos- phere can substantially increase the pheric scattering. Observations are extinction of light as it is transmitted conducted under cloudless, - through the atmosphere. Airborne less nights. Even distant or particulates, in the absence of light ground fog skew the results, ampli- pollution, can substantially reduce

Volume 18 • Number 4 2001 49 the faintest stars visible, even thought notice that the stars look sharpest the sky background may appear very and brightest in the late hours just dark (Garstang 1991). In this case, before . This trend is mostly the visibility of stars and astronomi- due to atmospheric turbulence which cal objects are lost to light scattering settles and diminishes as the night and absorption, not due to decreased progresses and the land cools. This contrast. Air pollution compounds trend may also be the result of re- the scattering of light pollution, fur- duced light pollution as people turn thering the degradation of night sky off their porch lights, park their cars, visibility. Finally, both factors are and outdoor athletic events come to a affected by humidity in the atmos- close. phere (Carr et al. 1989b). The growth and size of aerosol particles in the atmosphere is related to As with many natural resource moisture. Therefore, higher humid- measurements, much of the chal- ities are expected to exacerbate both lenge can be separating the natural the scattering of existing light pollu- and human components. Natural sky tion as well as the absorption of star- brightness does exist; the human- light (Garstang 1991). Conditions of made component of sky brightness is greater scattering tend to brighten light pollution. is the most nearby light sources while dimming obvious natural source, but can eas- far-off light sources (Carr et al. ily be avoided by sampling when the 1989b). The corollary to this phe- moon has set. , the nomenon is that dry, high-altitude spike of illuminated dust particles dark-sky sites are more susceptible to circling the inner solar system, can far-off light sources. be a significant natural light source. The lower atmosphere is turbu- It is most obvious in spring and lent, producing the common effect of autumn, but will set a few hours after stars. Turbulence scatters and rise a few hours before light and reduces the LM. Those dawn. Like the moon, zodiacal light precious few photons will be de- is easily avoided and simply results flected away from a single retinal cell, in a shorter observing window at and the eye will fail to detect a star, night (International Dark-Sky Asso- even though the night is dark and ciation 2000). pollution-free (Bortle 2001). There- is an important consid- fore, LM estimations will integrate a eration at dark-sky locations. This measure of atmospheric stability, results from the excitation of air when perhaps we are less interested molecules in the upper atmosphere in its effects than that of scattered that emit faint light. Airglow varies light or air pollution. Observers often with solar activity, and tends to be

50 The George Wright FORUM highest during the solar maximum class 1 skies are so rare now that few (which varies on an 11-year cycle; have ever seen them. the most recent was in 2001). Lastly, The definitions are galactic light and can be a included and cross–referenced with significant enough light source that LMs (Figure 3). The Bortle scale is they can affect both the eye’s ability suitable for a wide range of condi- to see faint objects and the brightness tions, from the brightest urban areas of the sky itself. Star counts within (LM lower than 4) to the darkest the Milky Way are more difficult due sites (LM up to 8)—an advantage to the glowing background of the over the LM star count method. The galaxy (International Dark-Sky Asso- LM star count is best suited to mag- ciation 2000). In extremely dark lo- nitude ranges between 5.5 and 6.5, cations, the brightest portions of the and is unusable with LMs above 7.2 Milky Way will create shadows and or below 4.5. The Bortle Dark-Sky can spoil the eye’s dark adaptation! Scale also integrates factors in a way However, for skies with LMs of 6.0 similar to our own aesthetic appre- or lower, airglow, galactic light, and ciation of the night sky. However, starlight are not a significant factor in this method tends to produce one total sky brightness. measure for an entire sky, as opposed to the LM star count method, which Although LMs have long been can produce multiple measurements used by serious amateur astrono- for different points in the sky, allow- mers, John Bortle recently proposed ing sky brightness to be associated a different, qualitative-based scale with directions or specific cities. It is (Bortle 2001). Built on the idea of also less able to capture finer varia- categorizing night skies (Schaaf tions in sky brightness. Bortle (2001) 1994), this nine-step scale has contends that the degree of human proved immediately popular, and has bias and error in LM star counts ex- a few advantages over the visual es- ceed the resolution of the method. timation of LM. Like the LM meth- However, basic tests using LMs ods, only a beginning knowledge of found adherence to the methods the night sky is needed. The Bortle produced an acceptable variation Dark-Sky Scale uses qualitative de- from observer to observer that is less scriptors to differentiate one class of than the 0.5-magnitude steps of the sky to another. For example, being Bortle scale (Moore and Duriscoe, in able to see the prep.). with the naked eye is indicative of class 6 skies and better. His scale is Dark night skies are an important based on 50 years of night sky ob- resource. Dark night skies are an air serving, and unfortunately his best quality-related value, and as such are

Volume 18 • Number 4 2001 51 52 The George Wright FORUM provided ancillary protection under quality of their dark sky resource. the 1977 Clean Air Act Amend- They are an effective first step in ments. They are an important com- monitoring and ultimately protecting ponent of wilderness areas in “re- this threatened resource. taining primeval character and influ- ence,” as defined in the 1964 Wil- derness Act. They are necessary to a In early 2000, the National Park growing list of wildlife species, and Service (NPS) funded a Night Sky are increasingly sought after by park Team. Using Natural Resource Pres- visitors as they lose the experience of ervation Program and Fee Demon- a starry night at their homes in the stration funds, the team set out to city. Night skies also serve as a “vital standardize methods for measuring sign”: an indication of the degree of and monitoring night skies, and to encroachment of development, and employ these methods at several of the level of cooperation between a parks. Complete methods for visual protected area and surrounding estimations of LM are available from communities. the Night Sky Team, and now in- The Bortle scale gives glimpses of clude cross-references to the Bortle the potential quality of night sky that Dark-Sky Scale. Additional quanti- is currently lost at most locations in tative methods and computer models the USA. Land managers whose re- are being refined or developed, and sponsibilities include significant will become the mainstay of the pro- night skies, but who have no measure gram’s efforts. In addition to stan- of their quality, are at least 30 years dardizing methods, the Night Sky behind. The value of a baseline con- Team provides technical assistance dition, even if substantial resources for community outreach and visitor have already been lost, cannot be interpretation. The Night Sky Team overestimated. Visual estimations of is based out of Pinnacles National night sky brightness are simple and Monument and Sequoia and Kings repeatable. Because of their simplic- Canyon National Parks, with support ity, they can “readily become the from the NPS Air Quality Division. most extensive body of data” avail- Several studies provide examples able on night skies (Kosai and Isobe of good science and leadership in 1992). They also allow a direct com- night sky management. In a program parison from one area to another, between amateur astronomers, the whether that area is a few miles away National Astronomical Observatory, or atop a distant mountain. and the Japan Environmental Visual estimations of night sky Agency, a map of sky brightness was brightness are an easy and rapid tool developed for the entire country of for land managers to inventory the Japan (Kosai and Isobe 1992). Com-

Volume 18 • Number 4 2001 53 bining star counts, photographs, and near and distant cities were mapped photometric observations, the study and their exact brightness values de- documented the location of the dark- termined. In 2001, these measure- est areas as well as the change in sky ments were repeated, giving the park brightness over time. long-term monitoring data and the Bryce Canyon National Park ex- ability to detect small changes in amined the potential impact from a night sky brightness over time planned coal mine, as well as the (Casper 2001). human perception of light pollution After only 18 months in opera- (Carr et al. 1989a, 1989b). These tion, the Night Sky Team (composed two studies were pioneering in their of resource scientists with other full- use of computer modeling of light time duties) is nearing standardiza- pollution. Additionally, they cross- tion of methodologies and referenced particular brightness val- completion of a pilot study at four ues in the sky to what is commonly national parks. The task remaining is perceived by park visitors. The hu- tremendous. At the time NPS was man perception of sky glow is an im- created, the night skies above our portant component of night sky pro- national treasures were unimpacted tection, since aesthetics and the wil- by light pollution. Today, only about derness experience are often cited as 1% of parks are free from this core values. problem. Many flagship parks in the In another study by NPS, Organ National Park System have Pipe Cactus National Monument substantial degradation, but fewer conducted complete sky surveys with than a dozen parks have any data a stellar photometer (NPS 1995). whatsoever on the quality of their Light pollution contributions from night skies.

Blackwell, H.R. 1946. Contrast thresholds of the human eye. Journal of the Optical Society of America 36:11, 624-643. Bortle, J.E. 2001. Introducing the Bortle Dark-Sky Scale. Sky and (February), 126- 129. Bowen, I.S. 1947. Limiting visual magnitude. Publications of the Astronomical Society of the Pacific 59:350, 253-256. Carr, E.L., et. al. 1989a. Impacts of the Proposed Alton Coal Project on the Night Sky Near Bryce Canyon National Park. SYSAPP-88/194.San Rafael, Calif.: Systems Applications, Inc. ———. 1989b. Evaluation of Night Sky Model and Human Perception of Night Sky Glow. SYSAPP-89/106.San Rafael, Calif.: Systems Applications, Inc. Casper, D. 2001. Personal communication. Planetary Sciences Institute, Tucson, Arizona. Hoffleit, E.D., and W.H. Warren Jr. 1991. The Bright . 5th ed. New Haven, Conn.: Astronomical Data Center and Yale University Observatory.

54 The George Wright FORUM International Dark-Sky Association. 2000. Night Sky Brightness Measurement Workshop. January. Tucson, Ariz.: International Dark-Sky Association. Garstang, R.H. 1989. Night-sky brightness at observatories and sites. Publications of the Astronomical Society of the Pacific 101:637, 306-329. ———. 1991. Dust and light pollution. Publications of the Astronomical Society of the Pacific 103 (October), 1109-1116. Kosai, K., and S. Isobe. 1992. Night sky brightness over Japan. Sky and Telescope (November), 564-568. Moore, C., and D. Duriscoe. In preparation. Night sky surveys at four California national parks. Paicines, Calif.: National Park Service, Pinnacles National Monument. NPS [National Park Service]. 1995. Night-Sky Brightness Monitoring Protocol for the Ecological Monitoring Program in Organ Pipe Cactus National Monument, Arizona. Ajo, Ariz.: National Park Service, Organ Pipe Cactus National Monument. Russell, H.N. 1917. The minimum radiation visually perceptible. Astrophysical Journal 45, 60-64. Schaaf, F. 1994. The seven layers of light. Sky and Telescope 88:5, 64-67. Walker, M.F. 1970. The California Site Survey. Publications of the Astronomical Society of the Pacific 82:487, 672-689.

Chadwick A. Moore, Pinnacles National Monument, 5000 Highway 146, Paicines, California 95043; [email protected]

Volume 18 • Number 4 2001 55