How dark is dark?

For years I have gone stargazing and made very off-the-cuff assessments over things like seeing and transparency. I have to admit that I've been too lazy to be a rigorous as I should be.

"Sure is dark tonight!" The engineer in me cringes when I say something like that. When someone says "it is cold," I want to know how cold: this is a question that is easily answered with a simple tool, the thermometer. Wouldn't it be nice to have something like a thermometer for measuring how dark it is – kind of like a light meter in reverse?

In fact, I bumped into a very nifty tool that does exactly that: the Sky Quality Meter from Unihedron. This measures how dark it is in magnitudes per square arc second. (mags/arcsec²). This is a quality, calibrated tool that is accurate enough to get comparison data from many sites. In fact, Unihedron has created such a worldwide darksky database.

There are several versions. The less expensive version measures a fairly large area of the sky, 42º. The more expensive version (by only $15), the Sky Quality Meter-L cuts that by half, making it a more accurate device, I think.

You might ask, "why not visually assess limiting magnitude?" I'll give a couple of answers to that. First of all, my aging eyes make that pretty limited and inaccurate. Secondly, a tool is both easier and more consistent. Finally, a tool like this is able to assess the darkness to a degree that not even the best human eyes could.