Western sky, large, ~1/6 moon diameter sustained flash, white and green low in the sky, moved downwards below the horizon very quickly.

Large flash, west from where I was, size is hard to say from a flash, maybe 1/6 of the moon across, possibly less, definitely larger than a regular star or planet. white in the middle, blue-green around the edges. When I noticed it it was low in the sky, I want to say about 15 degrees up. It was moving down very fast and went below the horizon about one second after I saw it. It was behind some dead trees, which made it harder to see. I hadn't seen it earlier because I was using binoculars and only saw it when I lowered them from my eyes, so I don't know when or where it first appeared.