﻿MODEEN 
  ASTRONOMY— 
  ABBOT 
  165 
  

  

  As 
  ozone 
  absorbs 
  to 
  some 
  extent 
  even 
  as 
  far 
  as 
  3,200 
  Angstroms, 
  a 
  

   catastrophe 
  might 
  ensue 
  if 
  circumstances 
  led 
  to 
  a 
  small 
  increase 
  of 
  

   the 
  ozone 
  content 
  of 
  our 
  atmosphere. 
  For 
  this 
  would 
  eliminate 
  

   indispensable 
  rays 
  of 
  wave 
  lengths 
  2,900 
  to 
  8,200 
  Angstroms. 
  Such 
  

   a 
  catastrophe 
  would 
  attend 
  such 
  an 
  alteration 
  in 
  the 
  distribution 
  of 
  

   the 
  extreme 
  ultra-violet 
  solar 
  spectrum 
  as 
  would 
  change 
  materially 
  

   the 
  existing 
  balance 
  of 
  influences 
  tending 
  to 
  produce 
  and 
  destroy 
  

   ozone. 
  

  

  This 
  leads 
  us 
  to 
  inquire 
  if 
  the 
  sun 
  is 
  a 
  constant 
  star, 
  and, 
  if 
  not, 
  

   what 
  is 
  the 
  character 
  of 
  its 
  variation. 
  The 
  fluctuations 
  of 
  sun 
  spots 
  

   and 
  other 
  visible 
  solar 
  phenomena 
  which 
  have 
  long 
  been 
  known, 
  

   prove, 
  of 
  course, 
  that 
  the 
  sun 
  is 
  not 
  absolutely 
  constant. 
  Within 
  the 
  

   last 
  22 
  years 
  the 
  Smithsonian 
  Institution 
  has 
  made 
  some 
  thousands 
  

   of 
  determinations 
  of 
  the 
  intensity 
  of 
  solar 
  heating, 
  which 
  prove 
  that 
  

   the 
  solar 
  radiation 
  increases 
  several 
  per 
  cent 
  at 
  times 
  of 
  maximum 
  

   visible 
  solar 
  activity. 
  Apparently, 
  too, 
  the 
  sun's 
  surface 
  presents 
  

   appreciable 
  inequalities 
  of 
  radiating 
  power, 
  so 
  that 
  the 
  rotation 
  of 
  

   the 
  sun 
  leads 
  to 
  successive 
  brief 
  changes 
  of 
  the 
  intensity 
  received 
  at 
  

   the 
  earth's 
  surface. 
  These 
  changes 
  are 
  slight 
  for 
  red 
  and 
  infra-red 
  

   rays, 
  but 
  grow 
  more 
  and 
  more 
  considerable 
  for 
  the 
  shorter 
  wave 
  

   lengths. 
  Pettit, 
  indeed, 
  observing 
  with 
  the 
  narrow 
  band 
  of 
  ultra- 
  

   violet 
  rays 
  which 
  silver 
  transmits, 
  centering 
  at 
  3,160 
  Angstroms, 
  

   finds 
  alterations 
  of 
  over 
  100 
  per 
  cent 
  in 
  their 
  intensity. 
  This 
  means 
  

   that 
  if 
  our 
  eyes 
  were 
  sensitive 
  to 
  such 
  rays 
  alone 
  we 
  should 
  find 
  the 
  

   sun's 
  surface 
  twice 
  as 
  bright 
  at 
  some 
  times 
  as 
  at 
  others. 
  

  

  Other 
  solar 
  phenomena 
  exhibit 
  interesting 
  variations. 
  Nearly 
  

   20 
  years 
  ago 
  Hale 
  discovered 
  magnetism 
  in 
  sun 
  spots, 
  and 
  later 
  over 
  

   the 
  sun's 
  whole 
  surface. 
  Sun 
  spots 
  are 
  apt 
  to 
  go 
  in 
  pairs, 
  and 
  Hale 
  

   linds 
  that 
  if 
  the 
  advancing 
  spot 
  of 
  a 
  pair 
  is 
  a 
  north 
  pole 
  in 
  the 
  

   northern 
  solar 
  hemisphere, 
  it 
  will 
  be 
  a 
  south 
  pole 
  which 
  leads 
  in 
  a 
  

   spot 
  pair 
  in 
  the 
  southern 
  hemisphere. 
  But 
  this 
  state 
  of 
  things 
  

   endures 
  only 
  through 
  one 
  11-year 
  sun-spot 
  cycle. 
  In 
  the 
  following 
  

   cycle 
  these 
  polarities 
  reverse, 
  so 
  that 
  221/^ 
  years 
  are 
  required 
  to 
  bring 
  

   back 
  the 
  magnetic 
  conditions 
  to 
  the 
  starting 
  point. 
  Bjerknes 
  has 
  

   proposed 
  an 
  ingenious 
  hypothesis 
  to 
  account 
  for 
  sun 
  spots, 
  their 
  cool- 
  

   ness, 
  their 
  going 
  in 
  pairs, 
  the 
  opposite 
  magnetic 
  polarity 
  of 
  the 
  pair, 
  

   and 
  the 
  reversal 
  of 
  polarity 
  at 
  each 
  11-year 
  cycle. 
  It 
  depends 
  on 
  

   hydrodynamic 
  principles 
  and 
  explains 
  the 
  phenomena 
  as 
  due 
  to 
  

   causes 
  residing 
  within 
  the 
  sun, 
  not 
  to 
  gravitational 
  influences 
  of 
  the 
  

   planets. 
  

  

  It 
  is 
  well 
  known 
  that 
  terrestrial 
  magnetism 
  reacts 
  to 
  solar 
  activity, 
  

  

  and 
  so 
  does 
  the 
  aurora 
  as 
  well. 
  Bauer 
  has 
  shown 
  that 
  the 
  earth's 
  

  

  magnetic 
  state 
  marches 
  closely 
  with 
  the 
  intensity 
  of 
  solar 
  radiation 
  

  

  as 
  measured 
  by 
  the 
  Smithsonian 
  Institution. 
  Very 
  recently 
  Austin 
  

  

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