Large star like object moving slow and vanishing.I just got finished bicycle riding. I live at the top of what is claimed the highest hill in Monroe CO. I keep a watch with me when I ride so I know what time it was. We were having high pressure so there was not a cloud in the sky all day.As I came to the top of the hill I saw what looked like Venus in the day sky (1hr+ before sunset). The sun was defininately the cause of the glow. It was moving what would seem as commercial aircraft speed heading directly away from me (I'm facing due west). I would have id'ed it as aircraft except for two problems. First, the craft left no vapor trail. Second, as the craft slowly became less reflectant by the sun as it moved away , I should have been able to see it better without the sun glarring off it. Before it stopped reflecting sun light back at me it vanished. If it was aircraft it would have been only on the other side of the city and I should have been able to see it. I feel the craft must have been very large and outside of the atmospher.