How Dark Is Your Sky?

Date: October 16-31
Time: 7:00 pm, one hour after sunset
Place: the eastern sky

Pegasus is the winged horse of Greek mythology. As with many other constellations, its fainter stars have been rendered invisible by light pollution. Only an easy asterism remains. Asterisms are subsets of the main constellation that look like something familiar, and for Pegasus that would be the “Great Square.”

At first glance, the inside of the Square appears devoid of stars. In fact, there are 13 stars in the Square brighter than magnitude +6.0, the normal limit for unaided, dark-adapted, human eyes. None of them are brighter than magnitude +4.0. By comparison, the stars that comprise the Square itself are around magnitude +2.5.

You can use the Square to do a simple test that measures the darkness of your night sky. Turn off all your outside lighting. Go outside and find a comfortable spot to observe the eastern sky. Give your eyes at least 15 minutes to adjust to the darkness. Relax. Enjoy the view of the sky as your eyes tune-in to the lower light level.

Now, count the stars you can see inside the Square and you’ll have a good measure of the quality of your night sky. Best to scan back and forth as you work your way down the Square. Be aware that, for faint stars like these, you’ll see them better when looking near them instead of exactly at them — a technique astronomers call averted vision. If you can see 7 of these stars, congratulations! You have a darker-than-average night sky. If you see less than 7 stars, you are living under light-polluted skies.

If you live in a large city, you will have light pollution. If you live outside the city, or in the suburbs, and still get less than 7 stars, it might be time to talk to your neighbors about their outside lighting. There’s a wealth of light pollution resources on my astronomy club website. You can access it here.

I’d be curious to hear from any readers who actually run this test. Describe your location and report how many stars you were able to count. Here in New River, Arizona, in the open desert about 30 miles north of Phoenix, on a good clear moonless night, I can count 10 stars in the Square. That’s pretty good for doing astronomy.

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