Black object hovering over central Phoenix

While walking my dog at about 6:40 or 6:45 pm on Friday, March 5th, 2010, I observed what I initially thought was a helicopter or balloon in the twilight blue sky directly and westward above me in north central Phoenix.  However, after I stopped walking to get a better look at the object, I realized that it wasn't moving, but rather drifting ever so slowly, and at times, it would remain stationary.  I stood staring at the object for a number of minutes trying to decipher what it was that I was looking at.  I thought to myself, is that a balloon?  I just couldn't figure out what it was that I was looking at.  It was just a sizable black object high in the sky, about the size of an aircraft, but shapeless.  It didn't make a noise, or have any lights, but it did seem to have a whitish hue or ambience to it at times.  After a number of minutes staring at the sky, I said to myself, I must look ridiculous, so I decided to move on, not to mention that my dog was ge! tting very impatient.  I probably wouldn't have thought much more about it, but what happened next is what dictated that I report it.  I looked away for only a second or two, probably took three or four steps, and then glanced back up at it, but it was gone.  It simply disappeared.  I immediately stopped again and stared in disbelief.  It was nowhere to be seen.  There were planes flying all over the place, but no black object.  I searched the sky for a number of minutes, but it was gone.  That's when I started to question the reality of what I just witnessed.  What did I see?  There has to be a rational explanation.