Astronomers Encourage Cities to Shield Outdoor Lighting

(Inside Science) -- Our home galaxy, the Milky Way -- that iconic stream of stars coursing across the night sky -- cannot be seen by one-third of humanity and 80 percent of Americans. As the artificial glow from towns and cities increases every year, and starry nights become unfamiliar to many, astronomers and dark-sky advocates are pushing to reduce light pollution -- starting with changes to outdoor lighting.

The American Astronomical Society passed a resolution at their annual meeting in Grapevine, Texas this month, “affirming that access to a dark night sky is a universal human right, making quality outdoor lighting a worldwide imperative.”

The organization also endorsed a set of recommendations for outdoor lighting. In short, “shield the light, dim it, and use redder, warmer colors,” said Lori Allen, director of the Kitt Peak National Observatory in Arizona.

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How to help reduce light pollution in three steps, following recommendations from the AAS .

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Light pollution from street lights, lit billboards and other outdoor light fixtures affects many people and wildlife species, nocturnal and diurnal alike. “It has huge effects on public health, the environment and certainly for astronomy,” said James Lowenthal, an astronomer at Smith College in Northampton, Massachusetts. Astronomers at observatories can see the effects of light glow from a city hundreds of miles away, to their dismay.

Many cities transformed their outdoor lighting following the recent revolution in light-emitting diodes, or LEDs. Blue LEDs earned three engineers the Nobel Prize in physics in 2014, since blue was the last color needed to make white LEDs. The new lights are now ubiquitous in smartphones, computer screens and energy-efficient light bulbs.

Nevertheless, these blue-white LEDs turned out to be worse for light pollution than the orange high-pressure sodium lights they replaced. Blue light at night can disrupt people’s sleep cycles by suppressing melatonin , a hormone that normally helps people sleep, more than other colors of light, said Martin Aubé, a physicist at University of Sherbrooke in Québec, Canada. Blue light also obscures people’s view of the night sky more than reddish lights.

The American Medical Association published a report last June -- which was endorsed by the astronomers’ resolution -- arguing that white LED lights have five times greater impact on circadian sleep rhythms than conventional street lamps. The LEDs are associated with reduced sleep times, dissatisfaction with sleep quality, excessive sleepiness, impaired daytime functioning and obesity, according to recent studies. In addition, the white LEDs contribute extra glare, putting drivers at risk.

Over the past year, so called low-temperature LEDs, which shine light with softer, yellower or redder colors, have been produced with the same energy efficiency and for similar prices as the bluer, less night-sky-friendly lights, said John Barentine, program manager at International Dark-sky Association in Tucson, Arizona. The color temperature of an LED, given in degrees Kelvin, describes whether the light it emits is more red or more blue. The higher the temperature, the bluer the light. Daylight-mimicking lights are above 4,500K, while 2,000K lights have a cozy, orange tint similar to the old sodium lamps.

Some cities and municipalities have been using this technological advance to reduce their light pollution. Flagstaff and Tucson, Arizona have long had light pollution ordinances, while Phoenix, the populous state capital, last year voted to replace all 90,000 lamps on streets and in parks with 2,700K LEDs, which have a yellower hue than their high-temperature blue-white counterparts. Montreal, Canada recently adopted a 3,000K standard, while Los Angeles, California is now doing the same for current modernizations. The city previously used higher-temperature 4,000K LEDs, which are less preferred by astronomers.

Most outdoor lights also can be dimmed by 25 percent or more without any loss of visibility, Aubé argues. The use of high-intensity lights in some areas can create the impression of a “lighting gap” as well, which can lead nearby areas to make their own lights brighter. A typical streetlight provides 50 times as much illumination as the full moon, but some lights are even more intense.

The third recommendation -- shielding light fixtures -- matters especially for astronomy and fans of the starry sky. Many street lamps project light upwards, but they can be shielded to prevent light from being emitted above the horizontal plane of the fixture. Lights with narrower angles further restrict the light glow above the city. But all outdoor lamps suffer from the problem of reflecting some light off the ground into the sky. Cities’ pavements and buildings are much shinier than the moon’s surface, Lowenthal said.

To get their night sky fix, many city dwellers escape to national parks. But even natural sites are not immune from light pollution. A survey of visitors at Bryce Canyon National Park and Cedar Breaks National Monument, both in Utah, found that 99 percent of them prefer to stargaze at a national park over other locations, 90 percent believe that there should be places that protect dark skies, and 80 percent thought that the surrounding communities should support such protections, said Ashley Pipkin, a biologist at the National Park Service in Boulder City, Nevada.

She believes that it’s important to maintain the view of the night sky. “It can help us understand our place in the universe and keep inspiring generations for years to come,” Pipkin said.