As I understood, the contraction affects only lengths in the direction of motion. And this is visible to a person who is at rest.
Then, there is a sentence on the book:
"Like time dilation, the length contraction is a reciprocal effect. To a person in a spacecraft, objects on the earth appear shorter than they did when he or she was on the ground by the same factor of [1-(v/c)^2]^1/2 that the spacecraft appears shorter to somebody at rest."
I don't understand what does Reciprocal effect means here ?
p.s this may be something obvious but I can't see the whole picture