Invisible aircraftIt was about 7:30 AM on the morning of the 15th of December, so the sun was just coming up and I was on the west-side of our building just looking up at the moon (I have become a pretty avid sky-watcher as of late and when you are alone it is just kind of enjoyable to look at the stars and moon etc.) and all of a sudden there were four lights just popped into the sky. They were in formation and equidistant from one another. They were flickering like what I would call afterburners on a jet from behind but there was nothing there. The only thing you could see is the four lights and they stayed exactly the same as it traversed from North to South right over us here. I watched thinking maybe I was looking at some birds that were reflecting the early morning sun, but then I saw a bird fly by and it looked totally different. These lights went South for about 20 seconds, not really fast but at a steady rate until they just poofed out of existence. I am absolutely certain that what I saw was not a natural phenomenon. It was four lights attached to something that was apparently invisible. Yesterday here in Denver Colorado it was a record cold day and I was actually wondering if maybe it was something we have built with some kind of new stealth technology but because of how cold it was out the exhaust was unable to be hidden.One last comment is that whatever I saw was close enough to me for me to notice that is made absolutely NO SOUND. I am going to watch with my camera in hand when I go outside from now on just in case I see it again I can photograph it for analysis. Oh, and another thing, I usually do not go outside at that time so maybe I had not ever seen it before because of the timing so I am going to go out at the same time again for a while to see if I can catch it. I will update with photo if I do get so lucky as to see it again. As of today I have not seen anything like it again.