Witnessing a triangular shaped object in the sky, travelling slow, low & silent with red,green,white lights on edges.

My neighbour, my husband , and myself were all in the backyard at around 10:00 pm at night. We were discussing building a fence later that summer.
 We saw this strange looking triangular object in the sky fly above us.  It was quiet, absolutely no noise heard from it.  It was travelling low and in a south-east direction.  It was about the size of the Good Year blimp but its shape was triangular.  It had about 20 lights all around its edges and they would flash in pairs in a synchronized pattern.  These lights were green, red  and white.  It was travelling slow and just glided over our heads.  It seemed like it was floating also like the Good Year blimp .
 It continued on its way and eventually was gone.  It was consistent in speed, not erratic but slow, and we wished it was not dark out to see clearly was this was.  This all happened in about a 3 minute period.
We will never forget it.  What baffled us the most was it being completely silent and the lights flashing in this unusual pattern.  We never saw anything like it again.  We still talk about it now and then.
Witness indicates that the date of the sighting is approximate.  PD