Saw an extremely bright light late in the afternoon traveling at a very high altitude.

I was sitting out on my terrace with three other people. My two year old daughter saw a balloon floating upwards. I looked at the balloon and watched it disappear behind our building. I looked up again a moment later to see if we could still see it but instead I saw a glint of an object extremely high up in the atmosphere. The sky was blue and our terrace is shaded in the afternoon so I was able to look straight up and follow the glinting object.
 At first I thought it might have been the balloon, but realized it was way too high but also flashing green, and traveling in an unusual pattern high above us. The object seemed to flash on and off as we followed it. It was moving north. I know this because it was moving in an uptown direction.
 At times I would lose sight of the object against the blue sky, but then it would flash and I would see it again. The light seemed to stay on as it traveled and I would momentarily lose it and then see it again until it finally just seemed to disappear upward.