XPost: alt.astronomy   
   From: henry@spsystems.net   
      
   In article ,   
   Earth Resident  wrote:   
   >How did scientists calculate the distance between earth and moon, earth and   
   >sun etc.   
      
   The distance to the Moon was pretty easy:  it's close enough that if you   
   observe it simultaneously from different parts of the Earth, its position   
   in the sky is a little different.  Knowing how far apart the observatories   
   are, and measuring the difference in apparent positions, you can compute   
   how far away it is.   
      
   Earth and Sun is a lot harder.  It's too far away for the same trick to   
   work very well.  Observational astronomy is all angles, so you can measure   
   *relative* distances to the Sun and the planets quite well, but unless you   
   can measure one of those distances by some other means, you don't know the   
   absolute scale of the system.   
      
   One of Captain Cook's voyages had, as its primary objective, timing a   
   transit of Venus (a rare event, Venus passing in front of the Sun) from   
   the other side of the Earth from Europe.  The difference in times would   
   give an absolute determination of the distance to the Sun.  It didn't work   
   too well:  because Venus's atmosphere blurs things, it's hard to decide   
   exactly when Venus crosses the edge of the Sun.   
      
   Various other approaches were tried.  The best of the pre-spaceflight ones   
   was that the asteroid Eros occasionally passes near the Earth, and by   
   looking at how much Earth's known gravity disturbs Eros's orbit in such   
   an encounter, you can determine how close Eros came.   
      
   Plans for planetary probes made the matter much more urgent, because they   
   need precision navigation, and so extra effort was put into the problem.   
   Shortly before the first Mariners were launched, the distance to Venus was   
   successfully measured by radar, reducing the uncertainty from tens of   
   thousands of kilometers to less than one kilometer.   
      
   To put the final icing on the cake, in the late 1970s, the distance to the   
   Viking landers was measured to within meters by timing signals they   
   received and relayed back.   
   --   
   MOST launched 1015 EDT 30 June, separated 1046,         | Henry Spencer   
   first ground-station pass 1651, all nominal!            | henry@spsystems.net   
      
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