Another UFO?

I was walking along Mill Lane tonight when something caught my attention in the northern sky. It was a bright green disc moving at considerable speed from right to left and on a downward trajectory, apparently heading for earth (although that would only certainly be true if the object’s course was at right angles to mine.) I saw it briefly through about 30° of arc before it disappeared behind some trees.

Whatever this thing was, it was far bigger than a meteor or a plane’s navigation light, and it was green. I’ve never seen anything like it before. Any ideas?