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  ANNUAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  farther 
  along 
  the 
  plane 
  of 
  the 
  milky 
  way 
  than 
  toward 
  its 
  poles, 
  or 
  

   else 
  a 
  pair 
  of 
  veils 
  of 
  progressively 
  increasing 
  obscurity, 
  like 
  two 
  

   caps, 
  one 
  on 
  either 
  side 
  of 
  the 
  galaxy, 
  hide 
  from 
  us 
  the 
  multitude 
  of 
  

   stars 
  which 
  otherwise 
  might 
  be 
  seen 
  toward 
  the 
  galactic 
  poles. 
  The 
  

   second 
  hypothesis 
  is 
  too 
  fantastic. 
  Astronomers 
  agree, 
  therefor'e, 
  that 
  

   OUT 
  system 
  of 
  some 
  SOpOOpOOpOO 
  stars 
  is 
  contained 
  in 
  a 
  lens-shaped 
  

   space 
  sotiie 
  five 
  times 
  as 
  extended 
  in 
  diameter 
  along 
  the 
  galactic 
  plane 
  

   as 
  at 
  right 
  angles 
  thereto. 
  

  

  2. 
  DISTANCES 
  AND 
  DIAMETERS 
  OF 
  STARS 
  

  

  When 
  Copernicus 
  conceived 
  the 
  sun 
  to 
  be 
  the 
  center 
  of 
  the 
  earth's 
  

   orbit, 
  and 
  the 
  apparent 
  motions 
  of 
  the 
  stars 
  to 
  be 
  merely 
  the 
  con- 
  

   sequences 
  of 
  the 
  earth's 
  daily 
  axial 
  rotation, 
  he 
  required 
  of 
  his 
  

   disciples 
  for 
  the 
  next 
  tliree 
  centuries 
  a 
  great 
  faith. 
  For 
  they 
  must 
  

   believe 
  that 
  the 
  stars 
  are 
  all 
  so 
  distant 
  that 
  the 
  immense 
  circuit 
  of 
  

   the 
  earth 
  about 
  the 
  sun 
  neither 
  seemed 
  to 
  bring 
  any 
  stars 
  nearer, 
  nor 
  

   even 
  caused 
  them 
  to 
  appear 
  to 
  approach 
  and 
  recede, 
  one 
  from 
  an- 
  

   other, 
  as 
  trees 
  do 
  when 
  we 
  pass 
  a 
  forest. 
  It 
  was 
  not 
  until 
  about 
  the 
  

   year 
  1840 
  that 
  the 
  most 
  refined 
  measurements 
  disclosed 
  any 
  angular 
  

   displacements 
  of 
  stars 
  attending 
  the 
  annual 
  revolution 
  of 
  the 
  earth. 
  

   Then 
  three 
  star 
  distances 
  were 
  measured 
  by 
  Bessel, 
  Henderson, 
  and 
  

   Struve, 
  respectively, 
  and 
  found 
  to 
  be 
  between 
  twenty 
  and 
  seventy 
  

   trillion 
  miles. 
  

  

  At 
  the 
  beginning 
  of 
  the 
  twentieth 
  century 
  less 
  than 
  100 
  star 
  dis- 
  

   tances 
  were 
  known. 
  About 
  that 
  time 
  a 
  new 
  method 
  was 
  introduced, 
  in 
  

   which 
  photography 
  was 
  substituted 
  for 
  eye 
  observation 
  at 
  the 
  tele- 
  

   scope. 
  By 
  dimming 
  a 
  bright 
  star 
  in 
  the 
  center 
  of 
  the 
  telescopic 
  field, 
  

   so 
  as 
  to 
  compare 
  equally 
  with 
  very 
  faint 
  stars 
  about 
  it, 
  we 
  may 
  ob- 
  

   tain 
  at 
  six-month 
  intervals 
  properly 
  exposed 
  photographs. 
  Since 
  the 
  

   very 
  faint 
  stars 
  will 
  almost 
  always 
  be 
  faint 
  because 
  of 
  immense 
  dis- 
  

   tances, 
  the 
  photographic 
  image 
  of 
  the 
  artificially 
  dimmed 
  star, 
  prob- 
  

   ably 
  much 
  nearer, 
  will 
  often 
  be 
  found 
  to 
  wander 
  very 
  slightly 
  to 
  and 
  

   fro 
  among 
  neighboring 
  star 
  images 
  in 
  the 
  six-month 
  intervals, 
  

   owing 
  to 
  the 
  earth's 
  annual 
  revolution. 
  By 
  assiduous 
  and 
  careful 
  

   investigations 
  of 
  this 
  kind, 
  which 
  have 
  been 
  carried 
  on 
  at 
  several 
  

   observatories, 
  these 
  paralactic 
  angular 
  displacements 
  have 
  been 
  de- 
  

   termined 
  for 
  nearly 
  2,000 
  stars 
  in 
  the 
  past 
  25 
  years. 
  Even 
  with 
  the 
  

   utmost 
  accuracy, 
  however, 
  it 
  is 
  impossible 
  to 
  do 
  more 
  than 
  show 
  that 
  

   a 
  star 
  is 
  relatively 
  a 
  distant 
  one, 
  if 
  it 
  lies 
  beyond 
  the 
  distance 
  requir- 
  

   ing 
  of 
  light 
  500 
  years 
  to 
  traverse. 
  

  

  As 
  comparatively 
  few 
  of 
  the 
  stars 
  lie 
  within 
  500 
  light 
  years' 
  dis- 
  

   tance 
  from 
  the 
  earth, 
  we 
  should 
  be 
  little 
  satisfied 
  unless 
  there 
  were 
  

   other 
  methods 
  of 
  extending 
  our 
  knowledge 
  of 
  star 
  distances. 
  As 
  

   concerning 
  measurements 
  of 
  the 
  distances 
  of 
  individual 
  stars, 
  there 
  

  

  