Purple Majesty of a Big Orb

Photo date: September 3, 2015. Photographer: Mark Mahin.

The photo below shows a school in New York City. I have taken quite a few photos at this exact spot, and have never had any other of my photos taken at this spot show an anomaly like this one: a huge purple orb on the left. There was no bright light nearby that might have caused lens flare (this is the full original photo).  The photos before and after showed no such anomaly.  See this series of photos describing nights on which I photographed this type of orb repeatedly, as many as 8 times on a single night, with the orb appearing in a wide variety of places.

There was no fog, mist, rain, or precipitation when I took this photo. The air quality listed on airnow.gov was "good," with a rating of 3 which indicates air about as clean as it ever gets (anything below 50 is considered good).