I have just started studying Special Relativity in class and the concepts of Proper Length and Proper Time are giving me some issues. I understand the basics of length contraction and time dilation, but the idea of this "proper" time and length is giving me problems. One example in particular that was bugging me: To start, the textbook I have for class defines proper length of an object as: "the length of the object measured by someone who is at rest with respect to the object." That makes sense, but in this example, it confuses me. It goes on to say: "Consider a spaceship traveling with a speed v from one star to another and two observers, one on Earth and the other in a spaceship. The observer at rest on Earth (and also assumed to be at rest with respect to the two stars) measures the distance between the starts to be L_p, where L_p is the proper length." It continues on to state that the observer on Earth observes the voyage taking Δt to complete and the observer on the ship observes the same journey taking Δt'. This makes sense to me due to time dilation, but the proper length confuses me. I understand that in this example, the Earth is at rest with respect to the two stars, but from the observer on the ship's point of view, would he too be at rest? To the observer on the ship, it would seem as if he is at rest and the stars are what is moving. Why does this disallow the observer on the ship to measure proper length, since, as the book suggests proper length is, he is at rest with respect to the object, it is just the object is moving to him in his reference frame. Any help would be much appreciated. Thanks in advanced.