How Dark Is Your Sky?

The left panel shows the northern sky and Little Dipper from near a city. Only the two brightest stars are visible. The right panel is how the sky looks from an outer suburb. Now you can see the Dipper's general shape, but one faint star in the Bowl's upper left corner is still missing. Maps created with Stellarium

How dark is your sky? For many of us that depends in which direction we look. From my home, I can see the Milky Way from north around to the southeast and halfway down the southern sky from the zenith. Forget about the bottom half of the southern sky. A dome of light pollution from the city of Duluth snuffs out the galaxy’s glory. Time exposure photos show the sky there tinged orange from a myriad of sodium vapor lights.

The entire outline of the Little Dipper is easily visible from the countryside; from a light pollution-free site, the number of stars swells exponentially.The faintest star or limiting magnitude most of us can see is around 6th. Under extremely dark skies stars of 6.5 are routinely seen. Some observers have even broken the 7th magnitude barrier!

There are few lights to the north, so the sky there is dark enough to see stars down to sixth magnitude and perhaps a bit fainter. Magnitudes are how astronomers describe the different brightnesses of stars. The system was developed around 129 B.C. by the Greek astronomer Hipparchus, who created one of the earliest catalogs of the stars. He called the brightest ones “first magnitude” because they looked “biggest” to the eye. The next level included the stars that weren’t quite as bright -  second magnitude -  all way down to sixth magnitude, the faintest stars Hipparchus could see. Before electric lighting became widespread, anyone with good vision and clear skies could routinely see stars down to sixth magnitude. Since most of us now live near cities, the average limiting magnitude (faintest star you can see) is closer to third or fourth magnitude. City dwellers are lucky if they get past first! How things have changed.

The difference between one magnitude and the next is a factor of 2.5, so a first magnitude star is 2.5 times brighter than a second and 100 times brighter than a sixth. With the invention of the telescope, Galileo expanded the magnitude scale beyond the six naked eye levels to around ninth magnitude, and the Hubble Space Telescope can reach 30th magnitude. On the other end of the scale, as the science of astronomy matured, astronomers began to realize that all bright stars aren’t created equal. Some are brighter than others. With nowhere to go but “up”, they extended the magnitude scale from first to zero and into negative numbers. Think of Vega and Deneb in the Summer Triangle. Hipparchus would have called them both first magnitude stars, but we know from looking that Vega is clearly the brighter. On the modern magnitude scale, Deneb shines at 1.2 (magnitudes are typically given to the tenth) and Vega at 0. The brightest star, Sirius, is magnitude -1.8 while the sun is -27.

The Little Dipper with magnitudes (to the tenth) shown for each of its stars. Since it has only three stars of third magnitude or brighter, it's no wonder most people have never seen the constellation. From a truly dark sky, you can around 2000-2500 stars on any particular night compared to only a couple hundred or fewer near a city.

So let’s get back to my original question – how dark is your sky? A good constellation to help you determine your limiting magnitude is the Little Dipper or Ursa Minor the Lesser Bear. It’s ideal for northern hemisphere observers, since it’s out all year and features stars with a wide variety of brightness. You can use the map above to find out. If you can see the two end stars (the Pointers) in the Bowl of the Big Dipper, they’ll point you to Polaris the North Star. From there you can work your way through the Little Dipper to see how many stars are visible. If you can’t see the Big Dipper, face due north and look up high to find the M of Cassiopeia, described and pictured in yesterday’s blog. Look two “Cassiopeia widths” directly below the M for a star of similar brightness to those in Cassiopeia. That star will be Polaris.

If you’re in or near a city, you might see second magnitude Polaris and Kochab, but that’s about it. Even from the suburbs, it takes some work to see all the stars in the handle, especially the upper left bucket star, which shines feebly at fifth magnitude. But look what’s in store if you observe from a truly dark sky. The whole area is alive with stars!

Once you get familiar with the magnitude scale using the Little Dipper, you can apply it to other parts of your sky to get a handle on how dark your location is – whether observing at home or away.

5 Responses

  1. Bibi

    What an awesome article, I loved it, thanks a lot Astrobob!!!
    Thanks to reading this, now I use the Little Dipper as a guide to tell me how clear or light-polluted my sky is on a particular day. Since I learned on here that this cool 7-star asterism has a variety of magnitudes (2,3,4 and 5) in it, I can pretty much just by checking it, know if my sky is worth or not observing that day. So far I’ve never been able to see naked-eye its faintest star, Anwar al Farkadain –a name I’ll never memorize, I guess LOL– but I keep trying, so one day I’ll “force” my eyes to see it even without binoculars! I already accomplished that with all its other faint stars, in fact most of the time I can barely see Yildun naked-eye not without much effort, and it’s a 4.4 mag star, which, in my totally light-polluted sky, and just a few degrees above the horizon to make it even more difficult, -when I see it these days it’s a bit lower than Polaris, which always is a fixed 20 degrees above the horizon in my sky — that’s quite a feat!!!

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  3. Learner

    Astrobob, great article! This is very helpful. I’ve been looking for a simple method to figure out how dark skies are, and this is exactly what I’ve been looking for. Thank you from a newbie astronomer! Clear skies!

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