I watched from our plane at 34,000 ft, a ball of light, about 4 times the disk size of the sun. The speed matched the plane and it was visiable for about 20 min off and on. It was below the plane and off to the side and as the cloud layer thinned or thickened it disappeared and you could see clouds pass between us. I thought it might be some kind of reflection because it looked like the sun, so bright, when seen through a fog. It finally disappeared. Nobody else on the plane said anything about it and when I mentioned it to a lady passenger she said she didn't see anything either. The flight was between Detroit and Seattle and I think the sighting was over Spokane approximately.