From within the plane, when you look down, why does it seem the plane is barely moving or moving at a snail's pace?

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This may seem like a stupid question and it might as well be, but I think about it every time I travel in an airplane.

As another user pointed out, the plane seems to be hardly moving because there are no reference points in the sky. You just look at a vast expanse of clouds and the blue sky. If there were mountains and rivers and skyscrapers hanging in the sky, you’d be able to see how fast the plane moves. That’s why you may notice that when a plane is about to land, everything seems to be moving extremely fast - that happens because at that altitude, you suddenly have a lot of reference points. And that’s how you get a glimpse of the speed of the plane when it’s about to land.