From: fortunati.luigi@gmail.com   
      
   The Earth takes 24 hours of the Earth's twin time for one complete   
   rotation on its axis.   
      
   How much time does the traveling twin (v=0.866c, gamma=2) take for the   
   same rotation?   
      
   Does it take 12 hours (24/2) or 48 hours (24*2)?   
      
   [[Mod. note -- A couple of comments:   
   1. I presume that in the 2nd sentence, the author actually meant to ask   
      how much time the travelling time *measures* for the same rotation.   
      
   2. The Earth's rotation period with respect to an inertial reference frame   
      is actually about 23 hours and 56 minutes.  Because the Earth is also   
      orbiting about the Sun in the same direction as its rotation   
      (counterclockwise when looking down from above the North pole), the   
      mean time from noon to noon is slightly longer, namely 24 hours.   
      The first image in   
        https://en.wikipedia.org/wiki/Sidereal_time   
      shows this nicely; in this context "the fixed stars" means an inertial   
      reference frame.   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)