0eb0861c   
   XPost: sci.physics.relativity, alt.astronomy   
   From: boyd@apt0.sao.arizona.edu   
      
   RichD wrote:   
   > How do astronomers know our galaxy is rotating, how do   
   > they measure the velocity?   
   >   
   > And, how do they measure the rotational velocity of other galaxies?   
   >   
   > --   
   > Rich   
      
   Galaxys are not solid disks.   From our vantage point within our galaxy   
     we can observe the differential velocity of  stars and gases  in the   
   arms and core using both optical and radio telescopes.  So we know the   
   direction of rotation and the velocity of many parts of our galaxy with   
   respect to us.   
      
   It's relatively easy to measure the rotational velocity of other galaxys   
   if their axis of rotation isn't pointed directly at at us.  Just lay the   
   slit of a high resolution spectrograph across the long axis of a spiral   
   or elliptical galaxy  and and measure the wavelength shift of the   
   ionized gases approaching on one side and receding in the other.  There   
   is a V*(sin (i))  correction there i is the angle of tilt of the galaxy   
   with respect to the viewer.  The measurement also shows the mass   
   distribution as a function of radius from the galaxy's center.   
      
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