Why do planes just sit in the sky?

An airliner in cruise is moving at around 900 km/h (490 kn), faster than almost anything you will ever see on the ground. Yet from a garden or a beach it seems to hang in the air. The reason is geometry. What the eye registers is not speed but angular motion: how quickly an object sweeps across the field of view. At a slant range of 15 or 20 km, a jet covering 250 m every second moves through less than one degree of arc per second. A car passing 50 m away at 100 km/h sweeps through many times that. The brain, used to judging speed from nearby objects, reads the slow angular drift as near-stillness.

The effect is stronger when there is nothing to compare against. Against a uniform blue sky there are no fixed landmarks, so the only cue is the aircraft's own tiny apparent size, and that hardly changes over a minute. Watch the same aircraft pass behind a tree or a chimney and its true motion becomes obvious at once.

Sometimes the illusion has a real component. Ground speed is airspeed plus or minus the wind. A light aircraft cruising at 150 km/h (80 kn) into a 100 km/h headwind advances over the ground at only 50 km/h, and a glider or a small trainer can genuinely creep. Near the coast, sea breezes and ridge lift let gliders and paragliders hold position for long periods, and helicopters and some vectored-thrust types can hover outright. Large jets, however, cannot: a Boeing 737 or Airbus A320 needs well over 250 km/h of airspeed to stay airborne, and even a strong jet-stream headwind rarely exceeds 200 km/h.

Another common sighting is an aircraft on final approach that appears frozen when seen head-on. If it is flying almost directly toward or away from you, its motion is along your line of sight and produces almost no angular movement; only its growing size betrays it. Airports with approach paths over suburbs generate this impression daily. The same aircraft seen from the side, at the same distance and speed, would cross the sky in a few seconds.

None of this changes the physics. The wing generates lift only because air is flowing over it, and the amount of lift scales with the square of airspeed. Below its stalling speed an aeroplane cannot stay aloft, so a fixed-wing aircraft that seems to sit in the sky is always moving through the air at high speed, whether or not it moves much relative to you.