flying plane's speed

[vishrut]

while flying at a very high altitude in an aeroplane, if we look outside through a window, we feel the plane is moving slowly and at the same time if we see another plane flying other direction, it seems to be flying faster than our plane. Why ?

[emeritus]

Put simply - Speed to the travelling observer is largely indiscernible without a reference point such as you have stated ( A passing plane).

[emeritus]

Its explained in more detail here

http://www.school-for-champions.com/science/motion.htm

[Clanad]

The closure rate (combined speed of both approaching airplanes) is about 900 mph...

[vascop]

The main reason is distance from the moving object. Let's say the plane is at 30,000 feet above the earth then when you look down at say two points a mile apart on the earth's surface, the angle between these points and your eye is 10 degrees.
Now consider 2 points a mile apart but only 500 feet from your eyes; in this case the angle is 160 degrees.
So this angle is 16 times the other. This means that you will appear to go past objects on the earth at a speed 16 times slower than objects only 500 feet away.
So a plane going past at 600 mph at a distance of 500 feet would appear to be travelling at about 37mph if it were travelling close to the earth's surface.
that's why when you look down, and if you can see the earth's surface, you appear to be travelling slowly.

[vascop]

By the way the speeds here are relative to the speed of the plane. Or if you prefer you can think of the plane as stationary for the purposes of illustration