eli5: Why do bigger aircraft appear to be flying more slowly than smaller ones both from the ground and from the inside?

Question

Every time I see a large plane in the sky, or even riding in one, it seems like it just flies more slowly. I wonder if it’s an illusion or do they actually fly more slowly?

In: Physics

Answer 1

It is more or less an issue of parallax. When something is closer to you it appears to move faster because you are seeing a smaller distance over a larger area. Hold a yardstick in your hand a foot from your face. The yardstick will occupy almost your entire field of view. Now stretch your arm out and the yardstick appears slightly smaller. Now place it on the far side of the room and it has gotten even smaller.

Larger aircraft fly higher in the air than smaller aircraft. Hold your hand out, the section of the sky your hand covers may be several miles across at distance the large plane is flying but may only be one mile or less at the distance a small plane is flying. Your perception of this is it takes the large plane 30 seconds to cross the distance of your hand so it must be moving slow. But the small plane crosses the distance of your hand in only a few seconds so it must be moving fast. The problem is your brain thinks they are both covering the same distance, the width of your hand, instead of the actual distance they are traveling. Your brain makes sense of that the best it can by saying the closer plane must be going a lot faster than the far away plane.

The same thing happens when you sit in a plane and look out the window. You look like you are barely moving because the section of ground you are looking at is tens of miles or more wide. Watch the ground as you take off and land. It will look like you are slowing down as you take off and gain altitude and speeding up as you lose altitude to land while the exact opposite is what is really happening. But because you are seeing more and more land area in the window during take off and less and less land in the window during landing, your perspective of your speed changes to make sense with how long it takes any visible item to cross the view of the window. When you are near land a visible item may only need to move a few hundred feet to no longer be in view versus when you are at cruising altitude a visible item may need to move 50 miles to no longer be in view.