I was sitting in my hot tub after work, enjoying a beer, which is my normal routine, on this warmer than normal spring night. i just happened to glance to the north and noticed a solitary dull white light moving in a due south path. i only observed the light for approximately 4 seconds before it went out. it’s speed seemed faster than i would consider an aircraft to be going, but not streaking across the sky. there were some broken thin wispy clouds over casper at the time and the moon was somewhat illuminating them. nothing so thick that it would conceal the light. there were no strobe lights of any color adjacent to this single white light. i continued to watch the assumed flight path with no sign of the white light, outline of an aircraft, no strobes and no sound. it vanished. my guess at an altitude would be around 20000 feet.