# Fast things from distance look slower than fast things in front of us, why?

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I’m baffled and don’t even know where to start thinking about this. Saw a commercial plane fly over today which must have been travelling at around 500mph and it looked so slow.

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It’s a phenomenon called parallax. Let’s start with a map, and a straight road. Pick a start and end point on that road. Now, in between that start and end point, pick a spot very close to the road, and a spot much further away. Now draw triangles between your start and end points, and the points you chose away from the roads, so that you have two triangles, one inside the other. Look at the agles between the close point, and the far away point. See how the triangle for the closer spot looks wider than the far away spot? That’s what your eye sees in reality too. The close object is moving along that wide short triangle while the far away spot is moving along the long skinny triangle. Thus, the closer the point is to you, the faster it seems to be moving.

A slightly more simple way to visualize this is by extending your arms out at a right angle, left arm pointing “half left” and right arm pointing “half right”. How far apart are the tips of your index fingers? About a meter / yard? So if something moves at 1m/s from one finger tip to the other, it will take 1 second to cross the whole distance.

Now look at what you’re pointing at at the horizon, let’s say 1km away. The point that your left arm is pointing to is about twice that far away from the point your right arm is pointing to, let’s say 2km. So if that same object from earlier was travelling at the same speed from one point to the other, it would now take 2000 seconds to get there. This is a long ass time when you’re holding your arms up, pointing like an idiot at the landscape. You’d probably think “move faster, you slowpoke!”

As you can see, travelling the same “apparent distance” appears much slower when far away, which is because the actual distance is much greater then.

Finally, consider the moon. You can block it out with your thumb, yet it is thousands of km across. If something was travelling the entire width of the moon in 1 second, it would look to you as if it was crossing your thumb in 1 second, so kind of slow, while in reality it would travel thousands of km in 1 second, which is really fast!