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Surveying Skyglow: satellite images don't tell the whole story--measurements by amateur astronomers are needed.

Anyone who observes the night sky knows all too well the telltale signs of light pollution. It might start as an isolated, bubble-shaped glow along the horizon--a "light dome" from some nearby town--that gradually and inexorably swells, broadens, and intensifies. In the worst cases, the pall of skyglow completely overwhelms the celestial scenery overhead.

Unfortunately, light pollution always seems to win. Or does it? Can anyone claim that the nighttime skyglow is actually lessening? Even if true, how could you tell?

Just recently, for the first time, I was able to spot the weak glow of gegenschein on a very clear night at my observing site in western Germany, one of the most light-polluted regions of the world. Other stargazers report seeing the Milky Way clearer than it appeared years ago. Are these improvements illusory, wishful thinking--or an attestable trend?

"Concerning worldwide changes of skyglow, there are large uncertainties on which direction we're moving," says Christopher Kyba (German Research Center for Geoscience), who has been studying light pollution as part of an interdisciplinary project called Loss of the Night. In fact, satellite images do suggest my observing site has gotten a little darker, but that's only locally.

Looking at the entire European continent from space, the changes are striking. With satellite data gathered over 15 years, Jonathan Bennie (University of Exeter, United Kingdom) and his colleagues recently found that although night brightness has grown significantly in most places, some large areas--and even entire countries--appear to be darker now than they were in the mid-1990s.

Meanwhile, a comparable study led by Christopher D. Elvidge of NOAA's Earth Observation Group seems to confirm this for other parts of the globe as well. His team finds that in some parts of the most developed countries, including the U.S., Canada, and Japan, night lighting overall either declined or remained stable between 1992 and 2012--despite growth in their respective populations and economies.

While these results raise hope for stargazers, Bennie cautions that apparent changes in brightness from satellite data (see the box on page 39) don't necessarily correlate with changes in ground-level light pollution. Still, if real, what causes these decreasing trends? And do they really bring back the stars?

The View From Orbit

Kyba's research involves quantifying and measuring light at night, and Germany's capital is an excellent place to do that. "Berlin is a large city surrounded by relatively dark countryside," he explains. "Compared to other metropolitan areas, you find all levels of illumination in a relatively constrained territory here." For example, just two hours' drive to the west is Westhavelland Nature Park, which in February 2014 became the first German dark-sky reserve recognized by the International Dark-Sky Association.

A Canadian physicist who gained experience taking extremely low-illumination images in particle physics and radiology before turning to light pollution, Kyba uses aerial photographs and satellite images to assess which types of lighting are best for preventing skyglow. He concurs with Bennie and Elvidge that some of the decreases measured by satellites result from better-shielded lamps that direct their light onto the ground and not into the sky. Less light is getting beamed into the sky.

However, looking down from space alone doesn't tell us the whole story, Kyba says. One limitation of satellites is that they measure light streaming directly upward, whether beamed in that direction or reflected off the ground. But this light is not what contributes most to skyglow. Rather, as shown in the simulation on page 38, worst is the light emitted just above horizontal, which can be scattered by air molecules, airborne dust, and water vapor for hundreds of kilometers in all directions.

For years researchers have struggled to calculate skyglow indirectly using the upward-directed lighting levels measured by relatively crude images from Defense Meteorological Satellite Program (DMSP) satellites.

In 2001, Italian astronomers Pierantonio Cinzano and Fabio Falchi published a highly useful global atlas of artificial night-sky brightness ( those maps plot the satellite-derived (not directly measured) skyglow intensities seen at ground level.

Newer spacecraft like the Suomi NPP and its visible-infrared imager (VIIRS) provide improved sensitivity and resolution, but there's a catch. Hundreds of millions of streetlights and security lights worldwide utilize high-pressure-sodium (HPS) bulbs, which emit a peach-colored light that's skewed toward red wavelengths. Future installations, both new and as HPS replacements, will be dominated by smaller, cheaper, and more energy-efficient light-emitting diodes (LEDs).

For astronomers, this could become a worrisome development. Most LEDs emit a large amount of blue light, which scatters much more readily than red light does and thus causes more skyglow. "To make things worse," Kyba explains, "both DMSP and VIIRS are not sensitive to blue wavelengths shorter than 500 nm. That's where LEDs have a big peak." (See the box on page 40.) In the worst case, these satellites might indicate decreased light pollution where there's actually been an increase.

Color photographs taken by astronauts aboard the International Space Station partly sidestep this wavelength blind spot, but those snapshots do not cover larger areas and are not taken consistently. "We need to do both," says Kyba. "Then we can get some ideas about how the ground illuminance is changing from the space perspective."

Here's Where You Come In

To really understand skyglow, we need to look not down but up. That's where amateur astronomers can play a crucial role. Instead of just guessing how bright (or less bright) their sky is, Kyba hopes more of them will actually measure it, helping him and other scientists get a grip on skyglow where it happens.

For example, Kyba's team has installed a small network of automated night-sky brightness meters around Berlin, but these cover a relatively small area. Similar detectors now continually measure the night sky over a handful of observatories and national parks in the United States. To increase the number of measurements, Kyba has launched a crowd-funded campaign to supply schools with portable, easy-to-use Sky Quality Meters (SQMs), converting schoolchildren to "citizen scientists."

But to acquire enough ground-level measurements to be scientifically useful, many more volunteers are needed--not just in Berlin but worldwide.

Fortunately, becoming a light-pollution scientist has never been easier. Over the past decade, thousands of amateur astronomers and other citizen scientists have submitted sky-brightness measurements to various projects worldwide. Three groups teamed up during 2009's International Year of Astronomy: "Globe at Night" (now managed by National Optical Astronomy Observatory, or NOAO), "Great World Wide Star Count" (National Earth Science Teachers Association), and "How Many Stars?" (started by Kuffner Observatory in Vienna, Austria).

All three efforts ask participants to do much the same activity: determine their sky's limiting visual magnitude by comparing a chart of certain constellations visible in the evening sky with the actual sky. The loss of limiting magnitude is still the best indicator for skyglow, especially if augmented with direct measurements acquired by digital light meters like SQMs.

With nearly 100,000 measurements from people in 115 countries during the last nine years, Globe at Night is the most successful of these projects--and these data are now being used for scientific studies. Students have produced four research papers as part of the NSF's Research Experiences for Undergraduates program, explains NO AO coordinator Connie Walker. These examined the effects of light pollution on the nocturnal flight routes of the lesser long-nosed bat (an endangered species) over Tucson, Arizona, and measurements of light over time above various local areas such as the region's mountaintop observatories.

Since 2014, Globe at Night has accepted observations year-round. The more who participate, the better. "To be most useful," Walker explains, "measurements should be taken from the widest possible range of locations--and also taken from the same location both seasonally and yearly."

To boost submissions, the Loss of the Night team created an app for iPhones and Android phones that guides observers to stars of descending magnitude visible at a specified time and place. It then asks whether each star can be seen. This app works best in bright settings where glare from the phone's display isn't a distraction.

Meanwhile, the Dark Sky Meter app, developed by Norbert Schmidt and colleagues at a Dutch software company in cooperation with the International Dark-Sky Association, uses an iPhone's camera to measure the sky brightness more objectively. The results are almost as good as those from SQMs. Finally, Globe at Night has developed an app that allows observers to submit naked-eye measurements in real time.

All these measurements get downloaded into the GaN database once each night, explains Walker, after which you can access the results using a regional map generated by the website. The regional-map function is extremely easy to use, and you can download the tabulated data for whatever area in the world you want.

These digital aids, in the hands of capable amateur astronomers, should help provide ample data for light-pollution research. Quantity is not quality, however --scientists need reliable data. Experienced amateur astronomers know that apparent skyglow depends strongly on cloud cover, dust and smog, atmospheric humidity, and of course moonlight. These factors are accounted for, but measurements are still done by untrained people with very different levels of experience.

In 2013, Kyba and colleagues from Italy and the U.S. published a paper demonstrating that the Globe at Night effort indeed provides useful data for tracking skyglow. However, they found that the range of individual observations is enormous--an average spread of 1.2 magnitudes--and that the spatial distribution of participants is far from ideal (most submissions come from densely populated areas). Still, thanks to Globe at Night's voluminous database, mean values of naked-eye limiting magnitude strongly correlate with both DMSP satellite values and the skyglow atlas by Cinzano and Falchi.

Conversely, of the more than 11,000 observations harvested by the Loss of the Night app in its first year, only 1,500 were suitable for analysis. Kyba explains that most participants did not know how to properly use the app or took their measurements under moonlit or cloudy skies.

This is why Kyba hopes to motivate amateur astronomers to participate. Although scientists are grateful for every observation submitted, those done by frequent stargazers--thanks to their familiarity with the night sky--offer the best quality. Besides, citizen scientists typically live in very light-polluted places, and most of them do their measurements only in the evening. "To find dark places in the countryside, it makes no sense to locate people only in city centers," says Kyba.

By contrast, amateur astronomers frequently observe from such dark places--and they're often out long enough to monitor skyglow throughout the night. (Sky brightness usually decreases after midnight, when most people are sleeping.) Amateur astronomers can help fill these gaps by providing measurements from times and places missed in current datasets, thus making them much more useful.

So the next time you're at your preferred stargazing site--whether it's your backyard or a remote park --why not take a few minutes to measure the sky's brightness? Whether you use an app, a light meter, or a tried-and-true "star count" doesn't matter, but your contribution is needed more than ever. It's especially important to get involved during the ongoing transition to LEDs, since backyard observers can provide crucial data that satellites will miss.

Who knows? Along the way you might find a new dark-sky oasis for your own enjoyment, even as you help to ensure that recent progress in the fight against light pollution has not been in vain.


If you're interested in making light-pollution measurements, here are links to key citizen-science efforts:

Globe at Night: globeatnightorg

Great World Wide Star Count:

How Many Stars?:

Dark Sky Meter (iPhone):

Loss of the Night App:

TEAM EFFORT More than 3,400 Indiana students participated in the Let There Be Night project, which yielded a light-pollution "map" consisting of 35,000 Lego blocks.

Measuring Trends in Europe's Light Pollution

Virtually everyone in the United States and Europe lives with some degree of light pollution, but some have it worse than others. Satellite imagery shows that truly dark skies are challenging to find in Europe.

More interesting is the map below, which shows how Europe's night sky has changed over time. To derive the changes, University of Exeter's Jonathan Bennie and others compared two 5-year compilations (1995-2000 and 2005-10) of DMSP images, which record light sent upward into space. Red hues denote increases in brightness and blue ones decreases.

For calibration, the researchers used a predominantly rural region in southwest England, where they found 111 isolated patches that have increased in brightness and 98 that had gotten darker. Among the latter, 15% were attributed to areas associated with industrial decline, while 45% were urban areas where streetlighting has been modernized.

"The decrease in lighting in urban areas is due to a combination of better shielding, more focused lighting, and some limited shutting off of lights," explains Bennie. "There may be some minor effects due to changes in lighting type, but LEDs have not been adopted on a large scale in the UK."

More generally, the United Kingdom shows mixed light-pollution trends, whereas Ireland, Portugal, and northern Italy have gotten considerably brighter.

Norway, Sweden, Denmark, Finland, and Belgium generally seem to emit less light now than they did in the 1990s. So do some Eastern European countries, most notably Ukraine and Slovakia. However, Bennie and his team suggest the underlying reasons have less to do with dark-sky awareness and more with industrial decline in the wake of the collapse of Communism.

Decreases in northwestern Europe call for other explanations. "In Scandinavia, a high degree of modernization of lighting has occurred recently," Bennie notes. But since LEDs have not been widely adopted, "my guess is that the decrease here is largely due to better shielding."

If there's a "loser" in the fight against light pollution, it's Spain, which generally has the most illuminated streets in Europe. According to a study of DMSP and Suomi-NPP satellite imagery by Alejandro Sanchez and colleagues from Universidad Complutense in Madrid, electrical consumption in public street-lighting almost doubled since 1990.

--Jan Hattenbach

LED Streetlighting: Promise and Pitfalls

We are on the cusp of a once-in-a-lifetime revolution in the way we illuminate our nighttime environment. Light-emitting diodes, or LEDs, are rapidly replacing any and all light sources used at night--from flashlights to headlights to streetlights.

There's much to like about LEDs. They are mechanically uncomplicated, produce a great deal of light from very little electricity, have extremely long lifetimes (up to 100,000 hours), and can be dimmed or cycled on and off instantly. Illumination technology has not taken such a dramatic step forward since the introduction of high-pressure sodium (HPS) bulbs in the 1960s.

However, white LEDs are strong sources of blue-rich light, which has several negative side effects. For example, blue-rich light is more disruptive to the circadian activity of nocturnal wildlife, including humans. As the graph shows, our eyes are much more sensitive to blue light at night than they are in daylight. Blue-rich light can create strong, often disabling glare within the eyes of elderly people. Most critically for astronomers, blue-rich light scatters readily in the atmosphere (which is why our daytime skies are blue). This means blue light creates far more skyglow at night than a similarly bright "warm" source such as HPS.

One easy way to determine an LED's apparent color is to note its correlated color temperature, or CCT. Very high values, 5000 kelvin or above, have the harshest, bluest light; those with CCTs of 3000 K or lower have a "warmer," more environmentally benign output. A 2010 analysis by the International Dark-Sky Association (IDA) addresses these problems in detail; see

The odds are high that your town or city is planning to convert its streetlight to LEDs, if it hasn't already. The same prospect applies to businesses near you. If so, the IDA makes the following recommendations:

* Always choose fully shielded fixtures that emit no light upward.

* Use "warm-white" or filtered LEDs with a CCT no higher than 3000 K.

* Choose models with adaptive controls like dimmers, timers, and motion sensors.

* Consider dimming or turning off the lights during overnight hours.

* Do not overlight just because LEDs have high luminous efficiency.

--J. Kelly Beatty

A physicist, passionate amateur astronomer, photographer, freelance writer, and newlywed, Jan Hattenbach is in the midst of moving from Germany to Chile.
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Title Annotation:Measuring Light Pollution
Author:Hattenbach, Jan
Publication:Sky & Telescope
Geographic Code:1USA
Date:May 1, 2015
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