How dark is dark? More so, in Saguaro Park's two units

2011-10-28T00:00:00Z 2014-07-22T11:24:18Z How dark is dark? More so, in Saguaro Park's two unitsTom Beal Arizona Daily Star Arizona Daily Star
October 28, 2011 12:00 am  • 

The intrepid BioBlitz volunteers who hiked the trails of Saguaro National Park last Friday didn't just count critters in the dark - they measured the dark itself.

About 70 volunteers carried Sky Quality Meters and took repeated measurements at regular intervals to measure the impact of light pollution on Tucson and on the east and west units of Saguaro National Park.

As you might expect, the sky gets a lot darker as you leave the city lights, becoming nearly pitch black in the canyons on the eastern side of the Rincon Mountains.

There, measurements at Happy Valley were basically as dark as it gets - 100 times better than in the middle of Tucson, said Constance Walker, a scientist who holds education posts with the National Optical Astronomy Observatory (NOAO) and the International Dark Sky Association.

Sky with a limiting magnitude of 2 was recorded in downtown Tucson. A person with good vision would see only 50 or so stars at that location, Walker said.

In Happy Valley, where the towering Rincon Mountains block city light, the measurement was 7. There, you could see up to 5,000 stars.

The higher the number in the measurements means a darker sky.

At Saguaro's west unit, where the smaller Tucson Mountains block much of the city's sky glow, measurements of 5 and 6 were recorded.

Across the city, measurements of 3 and 4 were recorded, "typical of a major-sized city," said Walker.

The sky darkens considerably on the outskirts, she said, a testament to local light-pollution ordinances. "Tucson is well-known for its well-shielded lights and low-pressure sodium (yellow) lights," she said.

NOAO, the National Park Service Night Sky Team and the International Dark Sky Association led the effort to measure the impact of light on the park units.

The Park Service Night Sky Team is dedicated to preserving the "natural lightscape" of national parks for visitors and for light-sensitive species.

Walker said the readings from the backcountry are a valuable addition to annual measurements made in populated areas of 115 countries during the international "Globe at Night" program, in which citizen scientists record sky brightness.

The BioBlitz, sponsored by the Park Service and National Geographic, was a 24-hour event that counted more than 850 species in Saguaro National Park's two units.

You can help

Through the annual Globe at Night program, you can help scientists measure the darkness of the night sky, beginning in January.

Go to: www.globeatnight.org

DID YOU KNOW

Flagstaff, in 1958, was the first municipality to adopt a lighting ordinance aimed at preserving dark skies. Tucson and Pima County adopted a comprehensive ordinance in 1972 and have updated it many times. Currently, Arizona and most of its counties have lighting ordinances.

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Contact reporter Tom Beal at tbeal@azstarnet.com or 573-4158

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