You have no true frame of reference for speed or motion in an empty sky. Aircraft moving toward you or away appear to be motionless. Something that looks to be a mile long might in reality be only a few tens of meters long when you have no idea how far away the object actually is.
True. And it happened really fast. Maybe the whole thing lasted 45 seconds.
Only one I saw was in Northern Nevada in the 90’s. A buddy and me were camping on the north east corner of pyramid lake on the shore. To the west above the “pinnacles” (prolly 8 miles away) on the north west corner of the lake I noticed strange lights hovering around, doing sudden zig zags, and all sorts of impossible looking maneuvers. It was only a few hundred feet off the ground. The lake is surrounded by mountains, so it is fairly easy to judge distance and altitude. I watched it in silence for a few minutes dumbfounded (I guess the imbibed substance starting with an L and ending with a D), didn’t help much, but my vision was confirmed when I looked at my friend, and he looked at me, and we both said simultaneously “Are you seeing what I’m seeing?”. My original theory was that perhaps it was a model helicopter (or perhaps an emergency helicopter from the hospital, people used to be allowed to camp there, and it was very possible for drunks to fall off the very tall rocks). It would be interesting landing there. The emergency helicopter was more likely as a model would have to be pretty big to appear that substantial from that far away. The other theory is that the atmosphere above a lake like that can refract light due to temperature differences between the air and the water, so things that appear in the sky can actually be on the ground, and it could have been a car approaching the pinnacles. The show was cool though.