What is Motion along the line of sight?

Motion along the line of sight is when something moves straight toward you or away from you, like a car coming down the road.

Imagine you're sitting on the sidewalk, and your friend is riding a bike directly toward you. As they get closer, they seem to grow bigger, that's motion along the line of sight. Now picture them passing you and then moving off into the distance. They look smaller as they go farther away, still motion along the line of sight.

Think about a train coming down the tracks. At first, it looks small, but as it gets closer, it seems to get bigger and bigger. That’s because it's moving straight toward you, along the line of sight. When it passes by and moves away, it looks smaller again.

It’s like looking at something that comes right up to your face and then goes back, just like when you bring a toy closer to see it better and then move it away!