﻿MODERN 
  ASTRONOMY 
  ABBOT 
  

  

  153 
  

  

  are 
  two 
  principal 
  methods 
  adapted 
  to 
  stretch 
  the 
  limit 
  of 
  measure- 
  

   ment 
  considerably 
  further. 
  The 
  first 
  depends 
  on 
  the 
  existence 
  of 
  

   numerous 
  pairs 
  of 
  stars 
  in 
  mutual 
  revolution 
  about 
  their 
  common 
  

   centers 
  of 
  gravity. 
  

  

  < 
  — 
  a3d<3^ 
  — 
  > 
  

  

  

  STraller 
  

  

  Via. 
  1.- 
  

  

  -Curve 
  of 
  light 
  variation 
  of 
  a 
  star 
  which 
  is 
  periodically 
  Gclipsed 
  by 
  a 
  

   larger 
  dark 
  body, 
  as 
  seen 
  from 
  tlie 
  earth 
  

  

  Although 
  too 
  near 
  together 
  to 
  be 
  distinguished 
  as 
  double 
  by 
  the 
  

   telescope, 
  the 
  duplicity 
  of 
  stars 
  is 
  sometimes 
  recognized 
  by 
  the 
  

   regularly 
  recurring 
  variation 
  of 
  their 
  light, 
  owing 
  to 
  their 
  mutually; 
  

   eclipsing 
  one 
  another. 
  In 
  many 
  other 
  cases 
  the 
  spectroscope 
  detects 
  

   duplicity 
  by 
  the 
  periodic 
  shifting 
  of 
  the 
  spectral 
  lines, 
  as 
  the 
  stars 
  

   tend 
  alternately 
  to 
  approach 
  and 
  recede 
  from 
  the 
  earth 
  in 
  their 
  

  

  