On Easter Sunday, April 12, 2009, I was sitting out front of the Caffe Trieste having a cappucino, in San Francisco's North Beach district. The location was 601 Vallejo Street, at the corner of Vallejo Street and Grant Avenue. My attention was drawn to one of the Caffe Trieste employees who was standing at the corner excitedly pointing up at the sky to a woman standing next to him. I got up and walked over to them and asked them what they were looking at.They pointed out a roughly circular object hovering in the evening sky. The time was exactly 5:40 PM, PST. I happened to have a compass in my laptop bag and quickly determined the azimuth to be 90 degrees due East. Later, using a simple protractor based inclinometer, I determined the angular elevation to be approximately 60 degrees plus or minus 5 degrees. The weather at the time of the sighting was clear, with some high altitude cirrus clouds. The wind direction and speed, according to the National Weather Service website at the time, was west to northwest, 7-10 knots, with gusts exceeding 15 knots.The object's angular size was approximately 1/4 Full Moon. It had a roughly circular shape. The color was a muted white, and it was clearly illuminated by the setting sun. The object did not appear to be illuminated from within, however. The object was completely silent and did not resemble any type of aircraft I'm familiar with.The object hovered in one position for a minute or so, before slowly moving in a southerly direction -- perpendicular to the direction of the brisk prevailing winds. As the object began to move it also began to change shape. It formed a rough triangle, and then became more of a rectangle. The other witness and I were loudly commenting on this shape change, when the object changed back into it's original circular shape. The object then appeared to recede, as though gaining altitude, while slowly moving to the south to approximately 110 degrees East Southeast, whereupon it simply faded from view.I turned another Caffe Trieste employee who had also been watching the object, and asked him what he thought he saw. At that moment, the first Caffe Trieste employee, who originally saw the object, exclaimed "There's another one!" The time was exactly 5:48 PM, PST.This second object was smaller, approximately 1/10 Full Moon in angular size and appeared in almost the identical 90 degrees azimuth as the first object. But the second object's angular elevation as considerably higher, approximately 80 degrees, plus or minus five degrees. This object hovered for about one minute before slowly moving to south and receding from view as if gaining altitude -- just as the previous object did -- until it was no longer visible to the naked eye.At this point there were a number people standing at the corner discussing what they had seen. I asked the first witness how he initially sighted the object. He said he'd been standing outside talking to a woman, when he noticed an object moving rapidly from the West to the East, when the object suddenly stopped and hovered. That's when he began pointing up at the object, and when I noticed and came over to see what they were looking up at. At that moment, a man sitting in his pickup truck across the street yelled"There's another one!" The time was exactly 6:05 PM, PST. This third object was identical in angular size to the second object, 1/10 a Full Moon. It's azimuth was approximately 130 degree East Southeast, and it's angular elevation approximately 80 degrees, plus or minus five degrees. This object was only visible for around 30 seconds before it rapidly receded from view, again as if gaining altitude.The entire incident spanned approximately 25 minutes. It was unfortunate that none of the customers sitting outside had cameras, nor were there any binoculars conveniently around. I immediately asked four of the witnesses if they would be willing to be interviewed about what they had witnessed. I am currently in the process of setting up these interviews. Three of the witnesses said they would have no problem recording the interview on video, while the fourth witness said he would consent to an audio interview.I was a combat photographer in the US Navy during the Vietnam War, and have extensive experience in identifying both private, commercial, and military aircraft. These three objects were clearly not aircraft of any type I'm familiar with. My immediate reaction when I initially saw the first object begin changing shapes, is that it had to be some kind of weather balloon, or balloon cluster. But after carefully observing the object alter its shape, I came to the preliminary conclusion that it was not a balloon or cluster of balloons. I based that preliminary conclusion on the fact that the wind was blowing steadily from the West to Northwest, and these objects both hovered and moved perpendicular to the direction of the wind. I still cannot rationally explain what I witnessed.It should be added that I am personally acquainted with all four of the witnesses, just in case this might impact my conducting a completely objective investigation.