What is it called when a plane looks still?

The Immobile Airplane Illusion: Understanding Parallax in the Sky

Have you ever been driving along and spotted a plane seemingly hanging motionless in the sky? It looks bizarre, almost defying the laws of physics. You might even wonder if it’s some kind of advanced aircraft testing a revolutionary form of anti-gravity. But the truth, as often is the case, is far simpler and far more fascinating: it’s all down to parallax.

The illusion of a stationary airplane, often observed near large urban areas like San Francisco, is a classic example of how perspective can deceive us. We experience parallax every day – it’s why objects appear to shift position when we change our viewing angle. Think about holding your finger out in front of your face and closing one eye, then the other. Your finger seems to jump against the background. That’s parallax in action.

In the case of the seemingly immobile plane, the effect is amplified by the vast distance between the observer (you, in your car or on the ground) and the aircraft. The plane is, of course, moving at a significant speed, but because of its considerable altitude, the angular velocity – the rate at which its position changes in our field of vision – is incredibly slow.

Imagine two points: the plane and a distant landmark like a mountain or a tall building. As you move, the landmark appears to move across your field of vision much more quickly than the distant plane. This is because the angular change in position of the nearby landmark is far greater than that of the plane. The plane’s distance creates a smaller apparent shift in position compared to the closer objects, leading to the sensation of it being stationary.

This effect is further enhanced by the plane’s relatively constant altitude. Unlike closer objects that might appear to speed up or slow down relative to your movement, the plane’s consistent height makes the parallax effect even more pronounced, reinforcing the illusion of stillness.

So, next time you witness this captivating illusion – a plane that seems frozen in the vast expanse of the sky – remember it’s not magic or advanced technology. It’s simply a delightful demonstration of parallax, a reminder that our perception of the world is constantly being shaped by our position and the interplay of distances and perspectives. The plane is moving, but the magic of perspective makes it appear beautifully still.