An effort is underway to quantify the sky glow above western St. Joseph County, IN, to establish its baseline status before the region is developed further. On June 5, 2018, I conducted a second sky survey near New Carlisle Economic Development Area using hand-held Sky Quality Meters (SQMs) and a Dark Sky Meter (DSM) app on my phone. The second survey yielded average SQM readings of 20.46 magnitudes per square arcseconds.
I started the survey just a few minutes before astronomical twilight ended at 11:24 PM, and concluded before the moon rose at 2:14 AM. The sky was clear, with only an occasional thin, small, wispy cloud. The six sites, depicted on the map, were similar to but not identical to the original five sites.
I did not spend a lot of time outside the car (I used the open sunroof to take readings), but an interim naked eye estimate southeast of Bendix Woods County Park suggests a limiting magnitude of 5.0. Using star charts of Hercules, I could discern the upper left star of Coronal Borealis (Iota CrB). A conversion formula suggests I should have seen fainter stars had I spent more time carefully observing. Also, naked eye measurements are subjective and dependent on one's vision.
The two SQMs, serial numbers 733 and 9896, yielded consistent measurements. The DSM, on the other hand, has greater variability and less reliability. The SQMs are dedicated light meters, whereas the DSM is a phone app. Hence, I put more value in the SQM numbers than the DSM numbers. That said, I did submit one roughly average DSM reading from each site to Globe at Night (GaN) using the Submit feature of the app, so those values should appear on the GaN map of results.
I recorded the original data in the field, then later entered the numbers into a spreadsheet that you can download.
By establishing a quantified baseline of the sky's darkness we can later measure whether and how much of the night sky is lost to additional light pollution. For example, this notion of measuring how much darkness is lost was conveyed with a physical model made out of 35,000 LEGO blogs from data gathered by 3,4000 students of the PHM School Corporation in 2009.
The SQM scale is logarithmic, but basically a pristine sky is a high 22 and an urban sky is around 17. In a blog post about a power outage in South Bend I note, "A meter reading of 17 corresponds to a naked eye limiting magnitude (NELM) of about three. That is, you can see down to third magnitude stars." In the region with SQM readings around 20, you can see stars fainter than mag=5. That's fairly dark and allows you to see the Milky Way, for example.