Large Manoeuvring Light which quickly accelerated out of sightTook Dog to lake for a swim, wife in Tow. At about six inches (at arms length) above the N.E. Horizon N54 28 30 W110 16 06 observed a large Bright Light travelling N.W. it's speed varied somewhat, then stopped, changed direction and proceeded out of sight quickly, becoming smaller until it was completely gone. this all happened within 2 minutes. I hold a Private pilots licence with a Night rating/endorsement and can comfortably state, "It wasn't an aircraft, Boy's!"