Was out in the garden getting my pet cat to come inside ,was looking at a bright planet in the night sky(think it was Venus), when i noticed what i thought to be a star beside it.It then started to move at a good pace across the sky, there was no sound ,it was very high up in the night sky and i thought it odd that it had no tell tale signs of aircraft navigational lights(no red lights or flashing lights).It did move at a speed and cover a distance much faster than a plane would, i watched it for about 3mins until it started to fade.This again was strange because it was still in view but it sort of was kind of dimming its light.The dimming was the only possible explanation because it still should have had the same illumination as it was still in view and there was NO cloud cover.