﻿PHENOMENA 
  OF 
  THE 
  ZODIACAL 
  LIGHT. 
  499 
  

  

  1831, 
  particularly 
  if, 
  as 
  it 
  has 
  been 
  said, 
  the 
  lightest 
  part 
  of 
  these 
  singular 
  twi- 
  

   lights 
  did 
  not 
  coincide 
  with 
  the 
  place 
  of 
  the 
  sun 
  below 
  the 
  horizon." 
  

  

  Sir 
  John 
  Herschel's 
  views, 
  published 
  only 
  five 
  years 
  ago, 
  were 
  called 
  forth 
  

   by 
  the 
  tail 
  of 
  the 
  great 
  comet 
  of 
  1843 
  having 
  been 
  by 
  some 
  so 
  pertinaciously 
  

   mistaken 
  for 
  the 
  zodiacal 
  light. 
  

  

  " 
  The 
  zodiacal 
  light," 
  said 
  he, 
  " 
  as 
  its 
  name 
  imports, 
  invariably 
  appears 
  in 
  

   the 
  zodiac, 
  or, 
  to 
  speak 
  more 
  precisely, 
  in 
  the 
  plane 
  of 
  the 
  sun's 
  equator, 
  which 
  is 
  

   7° 
  inclined 
  to 
  the 
  zodiac, 
  and 
  which 
  plane, 
  seen 
  from 
  the 
  sun, 
  intersects 
  the 
  ecliptic 
  

   in 
  longitude 
  78° 
  and 
  258°, 
  or 
  so 
  much 
  in 
  advance 
  of 
  the 
  equinoctial 
  points. 
  In 
  

   consequence, 
  it 
  is 
  seen 
  to 
  the 
  best 
  advantage 
  at, 
  or 
  a 
  little 
  after, 
  the 
  equinoxes, 
  

   after 
  sunset 
  at 
  the 
  spring, 
  and 
  before 
  sunrise 
  at 
  the 
  autumnal 
  equinox, 
  not 
  onry 
  

   because 
  the 
  direction 
  of 
  its 
  apparent 
  axis 
  lies 
  at 
  those 
  times 
  more 
  perpendicular 
  

   to 
  the 
  horizon, 
  but 
  also 
  because 
  at 
  those 
  epochs 
  we 
  are 
  approaching 
  the 
  situation 
  

   in 
  which 
  it 
  is 
  seen 
  most 
  completely 
  in 
  section. 
  

  

  " 
  At 
  the 
  vernal 
  equinox, 
  the 
  appearance 
  of 
  the 
  zodiacal 
  light 
  is 
  that 
  of 
  a 
  pretty 
  

   broad 
  pyramidal, 
  or 
  rather 
  lenticular, 
  body 
  of 
  light, 
  which 
  begins 
  to 
  be 
  visible 
  as 
  

   soon 
  as 
  the 
  twilight 
  decays. 
  It 
  is 
  very 
  bright 
  at 
  its 
  lower 
  or 
  broader 
  part 
  near 
  

   the 
  horizon, 
  and 
  (if 
  there 
  be 
  broken 
  clouds 
  about) 
  often 
  appears 
  like 
  the 
  glow 
  of 
  

   a 
  distant 
  conflagration, 
  or 
  of 
  the 
  rising 
  moon, 
  only 
  less 
  red 
  ; 
  giving 
  rise, 
  in 
  short, 
  

   to 
  amorphous 
  masses 
  of 
  light, 
  such 
  as 
  have 
  been 
  noticed 
  by 
  some 
  as 
  possibly 
  ap- 
  

   pertaining 
  to 
  the 
  comet. 
  At 
  higher 
  altitudes 
  its 
  light 
  fades 
  gradually, 
  and 
  is 
  seldom 
  

   traceable 
  much 
  beyond 
  the 
  Pleiades, 
  which 
  it 
  usually 
  however 
  attains 
  and 
  in- 
  

   volves 
  ; 
  and 
  (what 
  is 
  most 
  to 
  my 
  present 
  purpose) 
  its 
  axis 
  at 
  the 
  vernal 
  equinox 
  is 
  

   always 
  inclined 
  (to 
  the 
  nortward 
  of 
  the 
  equator) 
  at 
  an 
  angle 
  of 
  between 
  60° 
  and 
  

   70° 
  to 
  the 
  horizon 
  ; 
  and 
  it 
  is 
  most 
  luminous 
  at 
  its 
  base, 
  resting 
  on 
  the 
  horizon, 
  

   where 
  also 
  it 
  is 
  broadest, 
  occupying, 
  in 
  fact, 
  an 
  angular 
  breadth 
  of 
  somewhere 
  

   about 
  10 
  3 
  or 
  12° 
  in 
  ordinary 
  clear 
  weather." 
  

  

  The 
  ring 
  hypothesis 
  of 
  Cassini 
  has, 
  however, 
  been 
  followed 
  in 
  a 
  greater 
  or 
  

   less 
  degree, 
  by 
  La 
  Place, 
  Scherbert, 
  and 
  Poisson, 
  as 
  well 
  as 
  by 
  Humboldt, 
  

   who 
  is 
  an 
  observer, 
  and 
  publishing 
  in 
  1844 
  is 
  the 
  latest 
  of 
  all 
  the 
  authorities. 
  

  

  His 
  description 
  of 
  the 
  general 
  appearance 
  of 
  the 
  light 
  is 
  most 
  vivid 
  and 
  truth- 
  

   ful, 
  and 
  can 
  perhaps 
  only 
  be 
  fully 
  appreciated 
  by 
  those 
  who 
  have 
  seen 
  it 
  under 
  

   similar 
  favourable 
  circumstances. 
  

  

  " 
  Those 
  who 
  have 
  dwelt 
  long," 
  says 
  he, 
  " 
  in 
  the 
  zone 
  of 
  Palms, 
  must 
  retain 
  

   a 
  pleasing 
  remembrance 
  of 
  the 
  mild 
  radiance 
  of 
  this 
  phenomenon, 
  which, 
  rising 
  

   pyramidally, 
  illumines 
  a 
  portion 
  of 
  the 
  unvarying 
  length 
  of 
  the 
  tropical 
  nights. 
  

   I 
  have 
  seen 
  it 
  occasionally 
  shine 
  with 
  a 
  brightness 
  greater 
  than 
  that 
  of 
  the 
  milky 
  

   way, 
  near 
  the 
  constellation 
  of 
  Sagittarius 
  ; 
  and 
  this 
  not 
  only 
  in 
  the 
  dry 
  and 
  highly 
  

   rarified 
  atmosphere 
  of 
  the 
  summits 
  of 
  the 
  Andes, 
  at 
  elevations 
  of 
  thirteen 
  to 
  

   fifteen 
  thousand 
  feet, 
  but 
  also 
  in 
  the 
  boundless 
  grassy 
  plains 
  or 
  llanos 
  of 
  Venezuela, 
  

   and 
  on 
  the 
  sea-coast 
  under 
  the 
  ever-clear 
  sky 
  of 
  Cumana. 
  The 
  phenomenon 
  is 
  

  

  