I have written about how light pollution is reducing our enjoyment of the night sky. I have not written about how to determine night sky quality. There are fancy gadgets (meters) that can tell you about night sky brightness and then there are more subjective measures – your eyeballs.
We’ll start with your eyeballs; it is old school but valuable.
Two qualities are determined – Seeing and Transparency. On the surface they might appear to mean similar things, but you will see.
Seeing refers to the atmosphere’s stability. In a past article, I talked about how our atmosphere, while critical for life, can be disruptive for astronomy. When the atmosphere is stable stars do not twinkle and that is considered excellent seeing. For naked eye observing twinkling stars are not much of an issue, in fact they might appear dazzling, brilliant diamonds. But for observing with a telescope twinkling stars means whatever you are looking at will tend to pop into and out of focus a lot. It might look like it is jumping around too. Quite annoying.
There is a scale of seeing quality as published by the Astronomical League.
Excellent-no twinkling stars, period. Very Good-stars are twinkling slightly but brighter planet do not. Good-brighter planets twinkle slightly. Fair-brighter planets are obviously twinkling. Poor-everything is twinkling like crazy, forget about it. Of course, the very good, good, and fair assume there is at least one bright planet in the sky at the time you look. If there isn’t a bright planet (Venus, Mars, Jupiter, Saturn) in the sky it is trickier to make a seeing assessment. Here’s a tip – just go for stars twinkling only slightly. As I said this is subjective, based on the observer’s visual acuity.
Transparency is different. It is a measure of how stuff in the atmosphere makes celestial objects look dimmer, less distinct. This stuff can be humidity/moisture/high-thin clouds, smoke, dust, or light pollution. Another subjective measure based on the observers eyeballs.
There is a scale for transparency, published by the Astronomical League, using the Little Dipper for measurement.
1-you cannot see the star Polaris-BAD. 2-you can only see Polaris-STILL BAD. 3-you can see the two stars at the end of the Little Dippers bowl-FAIR 4-you can see a star in the Little Dipper’s handle-GOOD. 5-you can see six of the Little Dippers stars-VERY GOOD. 6-you can see all seven of the Little Dipper’s stars-EXCELLENT. 7-you can see stars near the Little Dipper that are not part of it-OUTSTANDING.
Funny thing about transparency. I my experience sometimes when the seeing is fair and there is a haze of moisture high up, this combo can improve planetary observing. The atmosphere seems steadier than it should be for fair seeing when there is upper-level moisture.
What’s in the Sky?
02/22; 3AM CST; south: A waxing gibbous Moon covers up (occults) open cluster M35 in Gemini.
02/24&02/25; pre-sunrise; east southeast: Mercury, Jupiter, and Saturn group up near the horizon.