I was on a late night flight from Washington to South Carolina, it was about 11pm, I was in a window seat. about an hour into the 2 hour flight, I looked out of the right side window towards the west. I cannot be sure exactly where we were, but I suspect we were somewhere over North Carolina. I could see many towns and I did see some small runways in the distance. It was a clear night, and in the distance I thought i could see some hills. I had been looking out the window almost the entire flight. In the distance, almost at the horizon line there was a large triangular object. At this time we were not flying over a very populated area. I could see little pockets of lights, some roads and I could see what I thought were hills in the background at the horizon line. From my perspective the triangular object I saw looked like it was lower in altitude than we were flying at, It looked like it was maybe 1km above the ground. The triangle object had a red light on the top tip of one of the corners, with red light down the side of it gradually getting fainter towards the other two corners of the triangle. It looked like a red search light with the light cone creating a pyramid. The shape was nearly a perfect equilateral triangle from my perspective. I first through this was a reflection of some light in the plane, but this was not the case as its position was in sync with the terrain and lights on the ground. Because of its apparent distance, and it being close to the horizon line I estimate it was really large. In comparison, runways that I had flown over earlier which were about the same distance were about the same length as the pyramid. Within about 10 seconds it started to get fainter, then seemed to turn so that the triangle object became thinner and thinner, and dimmer and dimmer, before disappearing totally.