Identifying Distant Light Pollution Sources

Ten years ago, I lived within easy walking distance of the south edge of Dodgeville, and on one starry evening, I walked to a favorite hilltop with a good view of the sky just south of town.  To my surprise and displeasure, I noticed a bright light dome in the southeast I had never noticed before.  Where was that light coming from?

Fortuitously, the bright star Antares was at that moment very close to the horizon, and right above the offending light dome!  I noted the time: 10:25 p.m. CDT on 15 May 2007.  And the observing location: 42° 57′ 06.4″ N, 90° 08′ 16.9″ W.

After getting home, I started up the Voyager planetarium software on my Macintosh, set the date and time to the observation time, and the observing location listed above.  I found that at that moment, Antares was at an azimuth of 134.2°.

Now, grabbing a protractor and a Wisconsin state map, I quickly determined that the most likely city along the 134.2° azimuth line from Dodgeville was Monroe, Wisconsin.  Though quite some distance away, could this have been the source of the light dome I saw?

Using a great circle calculation program on the internet and the known geographic coordinates (latitude, longitude) for the two locations using Wikipedia, I determined that Monroe is at bearing (azimuth) 133.5 from my observing location near Dodgeville at a distance of 35 miles.  This matched my star-determined azimuth quite well.

Was there an outdoor athletic event going on in Monroe at that time to cause so much light pollution?

Could the light dome possibly have been coming from Rockford, Illinois?  Even though Rockford’s bearing of 131.1° makes it a suspect, its line-of-sight distance of 71 miles makes this extremely unlikely.

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