From: fortunati.luigi@gmail.com   
      
   Luigi Fortunati mercoled=EC 27/09/2017 alle ore 00:01:01 ha scritto:   
   > The planet P distant 20 light-year, photographed with a telescope from   
   > Earth, appears as it was 20 years ago.   
   >   
   > An spaceship approaching P at gamma=4, while passing beside the earth,   
   > also takes a photo on planet P with the telescope.   
   >   
   > According to my calculations, in the spaceship picture, planet P appears   
   > as it was 160 years ago.   
   >   
   > Is my calculation accurate?   
      
   And if the spaceship is moving away from P (instead of approaching and   
   always traveling at gamma=4) in his picture, the planet P appears as it   
   was 2,54 years ago (about).   
      
   Correct?   
      
   --   
   Ce l'ho messa tutta ma non sono riuscito a costringere la mia ragione   
   ad accettare l'irrazionale.   
   Luigi Fortunati   
      
   [[Mod. note -- No, that's not correct, in fact as written your statement   
   isn't even *meaningful*.  The problem is the phrase "2,54 years ago":   
   in relativity there's no invariant way to compare time-at-place-A with   
   time-at-place-B, which is what you're trying to do.  In other words,   
   different observers (inertial reference frames) will get different   
   results for such a comparison.  So in order for such a statement to be   
   meaningful, you'd need to specify *which* observer (inertial reference   
   frame) you're referring to.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)