Ok. I've been reading about Bortle scale in different areas. Where do I find the Bortle scale for my city? I know when we lived in town you could only see, maybe, a few hundred stars if that, in the night sky. But when we moved on my mom's new farm next to the county line, it was almost a religious experience. It went from a few hundred to a few thousand. And I could see the Milky Way. Yes, I could see the faint glow from Owensboro and a few security lights from the neighbors but that's about it. And I have a clear sky chart bookmarked on my computer, Yeah, I know that's totally different, but I can't seem to pull up any data from it. What is going on?
When John Bortle first introduced his Dark-Sky Scale in the February 2001 issue of Sky & Telescope, it was reasonably well defined. It is based entirely on subjective criteria, designed to allow you to evaluate your own skies without using anything but your own eyes. Be aware, however, that different people may arrive at different Bortle estimates for the same site due to differences in eyesight and experience. Also, as Bortle originally introduced it, the Bortle Class was a property of a particular site. He has later clarified that he really meant it to apply to a particular night at a particular site -- in other words, the Bortle classification can vary depending on the conditions.
The original article is posted online here.
Since then, things have been somewhat muddied by the introduction of the various light-pollution maps that purport to predict the Bortle level for your site based on satellite data. The maps do not necessarily match reality perfectly, nor do they necessarily match each other. So when specifying your Bortle Class, it is useful to say where you got that number from.
Note that the maps are based entirely on skyglow from distant light sources. They do not and cannot take glare from nearby lights into account. If you are standing under the only streetlight in the middle of the Gobi Desert, your Bortle Class will be closer to 9 than to 1 -- as it would be if you were to walk a half mile away from that light.
To get from a Clear Sky Chart to a light-pollution map, click on Light Pollution map under Nifty Links directly below the colored bars.