I was sitting on a plane from Beijing to Hong Kong. While I was eating a meal, I saw another plane passed by. It was a small jet (something like Learjet 45) and flight toward opposite direction. It looked so close to me so that I was scared a while. After I finished the meal and a while, I realized that I am a physics graduate of great CUHK Physics Department, so I make the following order of magnitude estimation: 1.Width of the window : ~30cm 2.Separation between the window and me : ~1m 3.The plane appear in my sight for : ~0.5 second 4.Estimated plane speed : 900km/h Then Calculation: Relative speed between two planes = 900x 2 = 1800km/h Distance travel within time of sight = 1800 / 3600 / 0.5 = 250m Visual angle of me through the window = 30cm / 1m = 0.3 rad Separations between two plane = 250 / 0.3 = 750m It scared me a bit. Now I am busy to type and save this message to the laptop in case the authority can’t find the black boxes of both planes. -_-||||| | 