Say we have my friend blasting off in a spaceship to Planet Physics and I’m sitting here on earth. I think it’s quite obvious that I will measure the proper length between the planets (because I could use a meter ruler and measure it all) and that he will measure the contracted length (because his length is squished).
What I don’t understand is why does he measure the proper time of the journey and I don’t. I can appreciate that my friend should measure proper time because “he is stationary relative to both events (launch and touchdown)” but when I sit on earth I think I’m stationary to the launch and touch down too. So, why can I not measure the proper time it takes to reach planet physics? Can you explain why I am not stationary relative to touch down (I always see the planet staying still in the sky!)