Modern Physics - Length Contraction - Reciprocal effect

Question

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

Answer 1

The meaning of reciprocal effect is elaborate in the second sentance you quoted. That is, the person in the spacecraft observes the effect on objects on the earth and, reciprocally, an observer on the earth will observe the effect on objects in the spacecraft."

Answer 2

Special relativity – due to the Galilean postulate – is a theory with symmetric results. That is, what (time, length, mass, energy, etc.) is measured by one observer, it has the same magnitude from the viewpoint of the other observer while the observers are in motion relative to each other. In other words, if relativity predicts length contraction and time dilation from the standpoint of an observer, there is no place for length expansion and time shrinkage from the viewpoint of any other observer, otherwise it violates the symmetry of the theory.