ufo

I saw a UFO a few days ago.
I first saw it at about 60 degrees above the western horizon.
It was a small sphere (about 1/6 to 1/4 of the moon's apparent diameter) that seemed to turn from black to shiny at regular intervals.
A friend noted it was flying "a few hundred meters up, a km at most".
It was moving slowly towards the west, apparently descending; in about 3 minutes it disappeared behind the building.
Probably some bi-color party balloon drifting in the wind.