I was rather taken aback a few weeks ago when Kelly Beatty started his blog about Kitt Peak by asking, "How many of you can claim that your favorite stargazing spot is as dark now as it was five years ago, let alone 10 or 20?" — as though he was clearly expecting the answer to be "no."
In fact, I use several different observing sites, and I think they're all as dark as they were 5 or 10 years ago — so I sort of assumed that this is typical for the U.S. Northeast. In fact, I kinda think that my my country home is as dark as it was 35 years ago, when I first started seriously exploring the sky with 7×35 binoculars. But I strongly suspect that's not typical.
I actually have solid evidence from the last 5 years, ever since I purchased my Sky Quality Meter. I've taken hundreds of readings with it, and none of them show a clear trend of change over time.
This isn't entirely surprising, because to some extent light pollution is proportional to population, and most of the population growth in the U.S. over the past few decades has been in the South and the West. The Northeast has grown, but at a very modest pace.
Moreover, almost all the growth in the Northeast has been in middle and outer suburbs. The major cities all reached their current configuration in the 20s, and the inner suburbs in the 50s and 60s. And rural areas have, if anything, lost population.
So what about the mid and outer suburbs? Well, that's exactly where my astronomy club's observing field is. And there, also, I see no evidence of change over the last 5 years. (Click here if you can't see the graph at right.)
Round about now you may be wondering if all these Sky Quality Meter readings are just noise. Are there any meaningful patterns to be made out of them? Well, yes, in fact. When I plotted the measurements against time of night, the darkest readings are all around or after midnight. (Click here if you can't see them.) In case you're wondering — yes, I filtered out all the readings before the end of astronomical twilight. This effect is entirely due to decrease in artificial light. The same trend is discernible, though much weaker, at my country home. My guess is that it's partly due to commercial and home lights being turned off, but also very largely to the decrease in traffic. I strongly suspect that automobile headlights are the second biggest source of light pollution, exceeded only by streetlights.
But the most striking trend of all comes when I plot the measurements against time of year. The graph at right (click here if you can't see it) shows that darkness falls sharply after mid-October . . . just like the falling leaves on the trees, which are no doubt the primary culprit. That's not surprising at all, because New England is very heavily wooded. In the cities in particular, almost all the streetlights are directly under trees, which prevent their light from shining up into the sky. Conversely, the sky becomes much brighter when there's fresh snow under the streetlights. For more information on this effect, see my blog on the Snow-Corrected Light-Pollution Atlas.