Because your field of vision is a cone. Close up, you only see like a meter from left to right. So a close-by object doesn’t need long to pass through this distance. But in the far distance, your field of view covers kilometers. And naturally, traversing this long distance takes the objects there a while.
From your perspective it appears a if the objects move the same distance at different speeds, but it’s actually that they travel different distances at the same speed.
This is called “parallax” and is a method your brain uses to tell distances
One way you interpret motion is by the angular shift it makes. How quickly does it move from one side of your vision to another. This makes an angle, with your retina at the vertex.
If the object is close and moving, the angular change is very large in one moment, and so it’s interpreted as closer (all other things being equal).
If the object is far and moving it covers a smaller angle in your vision, and is interpreted as further away.
This is also the most direct way to measure the distance to stars 🙂 We know the speed of earth about the sun, the distance and timing. So we can look out at stars and see how they ‘appear’ to move. Larger angular differences are closer stars.
Latest Answers