You’re dozing off in the middle of a Gerard Butler movie as the ice slowly melts in the Bloody Mary before you. It seems, for all intents and purposes, like a lazy Saturday morning. But what’s actually happening is that you’re barreling through the sky at a rate of more than 500 miles per hour.

When you're traveling on an airplane, why does it feel like the plane is barely moving at all?

It’s partly because you’re moving just as fast as the plane is, which means you can’t track its progress by watching the plane itself. The same could also be said for cars, trains, and other vehicles—but when you’re moving in one of those, you often have a pretty good frame of reference. You can watch the trees zoom by your window or other cars disappear as you speed by them. The same can’t often be said when you’re traveling by plane: For most of the flight, you’re too far from the ground to use its fixtures as reference points.

If you’re close enough to the ground to see the plane’s shadow advance across the landscape, that can help you put the plane’s speed in perspective. Clouds can also help. “Another way to understand how fast you are moving is to note how fast thin, spotty cloud cover moves over the wing,” Sara Nelson, an aerospace educator and the director of the NASA Iowa Space Grant Consortium, wrote in an article for The Conversation. Just bear in mind that the clouds are moving, too, albeit at a much slower rate.

Now picture yourself on the ground, spotting a plane overhead. If there aren’t any clouds to use as reference points, it probably looks like it’s moving much more slowly than it is, too. But even if there are clouds, it still might seem like it’s soaring along at a snail’s pace. That’s simply because the plane is extremely far from you. As Nelson explained, “it takes longer for it to move across your field of vision compared to an object that is close to you.”

The illusion is probably for the best, as it might be a little alarming if we always perceived planes—from inside them or from the ground—to be shooting across the sky as fast as they’re actually going.

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This article was originally published on www.mentalfloss.com as Why Do Airplanes Feel Like They’re Moving So Slowly?.