The Bortle Scale: A Flow Chart
The Bortle Scale is a useful way of estimating your sky brightness, i.e. to what extent light pollution affects your view of the night sky. By going outside on a clear moonless night and recording what astronomical objects you can see you can assign a Bortle Class rating to your observing site.
I have used the Bortle Scale to assess night sky quality many times, and always felt the lack of a handy flow chart to lead me through it. So I made one. Enjoy. (You can also download the pdf version.)
PS The content of this chart assumes some prior knowledge of astronomy, but any of the terms used are easily google-able.
Categories: Dark Places, Light Pollution, Stargazing
astronomy, Bortle Class, Bortle Scale, dark skies, dark sky, Light Pollution, M31, M33, milky way, sky brightness, Zodiacal Light
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Hi,
John Bortle, the creator his Dark Sky Scale, says that his scale is not intended for use in association with any map, including the World Atlas referenced above. It is to be used, as you say above, when on-site to evaluate current conditions. He says any such map correlation will often be misleading and have significant error. In other words, out of respect for the creator’s wishes, please remove the reference to the World Atlas (and the colors, which confuse people by association). To verify, visit the Cloudy Nights forum, and look in the Light Pollution forum. John Bortle of Brooks Observatory (his private observatory) posts there himself. There is a current thread “Accuracy of Bortle Scale” which shows the confusion from connecting these, and Mr. Bortle’s own comments.
Regards!