We were just coming back into town on Broadway, which at the time was overhung by elm trees on both sides. A couple of blocks in, I saw this ten foot or so circle of blue light float over the opening in the trees, moving from NE to SW. Maybe going about twenty mph at a hundred feet or so. It was an overcast night, and the streetlights weren't all that bright on that street, so the motion of a different color light caught my eye. I didn't say anything at first, but the girl with me said, "Did you see that?" We never were able to guess what it was. The most 'likely' that we could come up with was maybe some kind of airplane, but I don't think there are any blue lights on planes. Like I said, all we saw was the thing float through the gap between the trees... Neither of us were all that upset or scared, just wondered what the heck it could of been.