As twilight deepened after sunset last Sunday, I was greeted with a darkening sky of jaw-dropping clarity. You know how it is — when the winter stars seem so bright and close that you can't take your eyes off them.
As it turns out, that was Day Two of this year's Globe at Night (GaN) campaign. Each year since 2006, amateur skywatchers around the world have taken stock of how dark their skies are by eyeballing the stars of Orion and comparing what they see with simple star charts. The resulting magnitude estimates provide a snapshot of how dark the night sky looks from backyards around the world.
Globe at Night is like an annual light-pollution checkup. I've been a dedicated participant in this effort, because I'm committed to keeping my observing site as dark as possible. In years past I've strained and squinted to glimpse every last star possible in the Orion region. I'm luckier than most: usually my backyard boasts a fairly dark limiting magnitude — my very best night ever was 5.4.
Sunday night was windy and bitterly cold, a climatic combo that seems to go hand in hand with pristine winter skies ("Why can't it be this clear in September?" I mutter to no one in particular.) So instead of gauging the stars by eye, I whipped out my Sky Quality Meter, a nifty hand-held gizmo that gives an instantaneous reading. It gave a reading of 19.38 magnitudes per square arcsecond, which corresponds to a limiting magnitude of 5.1. Nice!
Last year, Globe at Night's "citizen scientists" tallied 14,249 measurements from 115 countries. This year's effort, with a goal of at least 15,000 reports, has four observing windows: January 14-23, February 12-21, March 13-22, and April 11-20. "For the January through March campaigns, we will use the constellation of Orion," explains Connie Walker, who coordinates the program. "For the April campaign, we are going to use the constellation of Leo in the northern hemisphere and Crux in the southern hemisphere."
So what about you? Haven't you wondered how dark your sky is — and how many stars you've lost to light pollution? Joining Globe at Night is easy — no prior stargazing experience is necessary! Just download the comparison charts (for Orion, Leo, or Crux). spend at least 10 minutes out in the darkness (so your eyes can adapt), make your estimate, then report the results.
That's it! In less than a half hour, you'll not only know your backyard's rating but also have contributed to a global campaign to save the night sky. Won't you join me in this noble effort?