Light Pollution & Stargazing Calculator
Estimating your sky darkness usually means cross-referencing a map and squinting at it. Instead, count how many stars you can see in the Little Dipper and get an instant Bortle class. Works for Northern Hemisphere observers. Nothing uploaded.
Result
Bortle 8Sky description
City sky
Notes
This method requires the Little Dipper to be above the horizon and the sky to be clear of clouds and haze. It is an approximation, not a precise sky-quality meter reading.
Learn more: the Little Dipper darkness method
Counting stars instead of reading a light meter
The Little Dipper has seven main stars of very different brightness. Wikipedia's Ursa Minor article lists Polaris at about magnitude 2.0, Kochab at 2.08, Pherkad at about 3.04, Epsilon at 4.22 and Yildun at 4.35, with the last two at about 5. A lower magnitude means a brighter star. The more of them you can see, the fainter a star your sky lets through.
Skywatch columns that teach this method use the bowl, whose four stars run from magnitude 2 to 5. One version says that seeing all four means a good, dark sky, and that seeing only Kochab and Pherkad, the two Guardians, ranks the sky fair to poor.
What the tool's table says
The tool maps your count to a limiting magnitude, using the seven stars' brightness. Seeing 3 stars, down to Pherkad, means about magnitude 3.0. Seeing 4 means about 4.2.
It then converts that to a Bortle class using the scale's limiting magnitudes. On the Bortle scale, class 9 is 4.0 or brighter, class 8 is 4.1 to 4.5 and class 7 is 4.6 to 5.0. Class 1 starts at 7.6, and class 3 is 6.6 to 7.0.
Seeing all seven stars gives "about magnitude 5.0 or fainter" and "Bortle 7 or darker". The faintest stars in the Dipper are about magnitude 5, so the test cannot tell you how much darker than that your sky is. For a darker sky, count stars in a fainter pattern, or use a sky quality meter.
The scale and its limits
Bortle introduced the scale in the February 2001 issue of Sky & Telescope. This is a rough field method and not an instrument reading. Moonlight, haze and a Dipper low on the horizon all hide faint stars, so use a moonless, clear night with the Dipper well up.
FAQ
Why the Little Dipper?
Its stars cover a range of brightness from about magnitude 2 to 5, and it sits near the north celestial pole, so it is up all night from mid and high northern latitudes. The tool is not meant for the Southern Hemisphere.
Does moonlight change the count?
Yes. A bright moon lights the whole sky and washes out faint stars, the way light pollution does. Count on a moonless night to measure your site's own darkness.
What limiting magnitude goes with each Bortle class?
Wikipedia lists class 1 as 7.6 to 8.0, class 2 as 7.1 to 7.5, class 3 as 6.6 to 7.0 and class 4 as 6.3 to 6.5. Class 5 is 5.6 to 6.0, class 6 is 5.1 to 5.5 and class 7 is 4.6 to 5.0. Class 8 is 4.1 to 4.5 and class 9 is 4.0 or brighter.