National Geographic Society Newsroom

Counting Stars at the Saguaro National Park BioBlitz

  By Sean O’Connor Artificial lights flood the night sky, making the urban and suburban lives that so many of us live a little brighter, but not necessarily for the better. Light pollution also drowns out the sea of stars shining through our atmosphere. Who doesn’t love to look up at the night sky and...

Two hikers taking SQM measurements of the night sky on a trail in Saguaro National Park East. Photo courtesy Jeremy White, National Park Service


By Sean O’Connor

Artificial lights flood the night sky, making the urban and suburban lives that so many of us live a little brighter, but not necessarily for the better.

Light pollution also drowns out the sea of stars shining through our atmosphere. Who doesn’t love to look up at the night sky and see a palette full of bright, shining, stars? How many of us perhaps have never experienced this at all because of where we live?

But our human desire to experience the unadulterated night sky isn’t the only reason to care about the brightness of lights at night. There are also animal species that can be negatively affected by these glowing bulbs. That is why the GLOBE at Night program coordinated a Tucson-wide night sky inventory during the 2011 National Geographic BioBlitz in Saguaro National Park.

Connie Walker of the National Optical Astronomy Observatory helped coordinate the inventory event. To measure the night sky brightness, a type of instrument called sky quality meters were used by all the groups of people who hiked across various trails in both parks as well as drove across four major streets of Tucson, Walker explained. “Some drivers took measurements through their moon roofs. During the daytime participants in the BioBlitz event catalogued over 850 species of animals and plants. The night sky inventory data will be used to compare with these species,” Walker said.

According to Walker, one such comparison studies how light pollution impacts lesser long-nosed bats in the Tucson city area to see whether or not light pollution is affecting their foraging habits. Preliminary results show that the bats seem to preferentially travel in the darker areas of Tucson to get from roosts in Saguaro National Park East to foraging areas.

Overall, the night sky inventory during BioBlitz 2011 in Tucson  “was a tremendous success,” says Walker. Values ranged from a stellar magnitude of 2 at the center of town to 6 at the far edges of the national park.

Results show that the skies are tremendously dark at Happy Valley Saddle in the Rincon Mountains (in the more remote East district of Saguaro National Park). A limiting stellar magnitude of 7 (e.g., the faintest star you can see) at Happy Valley means that the night sky is basically unpolluted. You can see thousands of stars there as opposed to downtown Tucson, where you see less than 50 with a limiting stellar magnitude of 2.


In the map above, the lighter the dots, the brighter the night sky. The darker the dots, the darker the night sky. Map courtesy of Mark Newhouse and Dave Bell, NOAO

This practice of measuring stellar magnitude, or the brightness of stars or other celestial bodies from Earth, dates back to a period when light pollution was not an issue at all, over 2,000 years ago, even before the time of the astronomer and geographer, Ptolemy.

The GLOBE at Night program is an international citizen-science campaign to raise public awareness of the impact of light pollution by inviting citizen-scientists to measure their night sky brightness and submit their observations to a website from a computer or smart phone.

According to the GLOBE at Night website, “light pollution threatens not only our “right to starlight, but can affect energy consumption, wildlife and health.”

The GLOBE at Night campaign has run for two weeks each winter/spring for the last six years. People in 115 countries have contributed 66,000 measurements. The night sky inventory in Tucson was coordinated by the NOAO Education and Public Outreach group in collaboration with the International Dark-Sky Association and the National Park Service’s Night Sky Team.

Join the GLOBE at Night program as citizen scientists in 2012 on the following dates: January 14-23, February 12-21, March 13-22, April 11-20.

Sean O’Connor is the project coordinator of educational mapping for National Geographic Education. When he’s not creating maps or advising his colleagues on mapping issues, he enjoys researching history, canoeing and kayaking, and exploring the world around him. In his work at NG, Sean has helped to develop the National Geographic FieldScope tool and launch a new suite of dynamic, on-line mapping tools for students and teachers.

About National Geographic Society

The National Geographic Society is a global nonprofit organization that uses the power of science, exploration, education and storytelling to illuminate and protect the wonder of our world. Since 1888, National Geographic has pushed the boundaries of exploration, investing in bold people and transformative ideas, providing more than 14,000 grants for work across all seven continents, reaching 3 million students each year through education offerings, and engaging audiences around the globe through signature experiences, stories and content. To learn more, visit or follow us on Instagram, Twitter and Facebook.

Meet the Author

Author Photo David Max Braun
More than forty years in U.S., UK, and South African media gives David Max Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. In his 22-year career at National Geographic he was VP and editor in chief of National Geographic Digital Media, and the founding editor of the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directed the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. A regular expert on National Geographic Expeditions, David also lectures on storytelling for impact. He has 120,000 followers on social media: Facebook  Twitter  LinkedIn