On my diagram, a person is travelling in a spaceship to the right in red, which is displayed as a Minkowski diagram . At the origin (0,0) the person travelling travels by a stationary person. And the 2 blue lines are the worldlines of two blue stationary rocks.But when I try to figure out what the distance is between the rocks from the frame of reference of the spaceship at (0,0), I get a distance wider than the proper length. What am I doing wrong?