Was shocking at first, not knowing what it was but when it left I had a very good idea

It was just getting dark and we had been watching planes come in and out of our area. I then saw a flash of green light come down (quite far away in the distance, a few KM] and it quickly moved horizontally to the left. Then it stayed stationary for almost a minute with a blinking greenish light. Before zipping off to the right more than twice as fast as any other plane seen and disappearing. It wasn't until this happened that I thought of it to be a UFO. I have never seen anything like this before and thought I never would. I don't know what it was, but I do know what it was not (a plane, a flare, a weather balloon, a satellite or any shooting star, etc) . Because it was quite far away, like the distance we had watched the other planes, and because it was getting dark, it was hard to determine the exact shape. But when it took off, it looked to have been a disc like shape.