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librationmore about libration


  1  definition  found 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
  Libration  \Li*bra"tion\  (l[-i]*br[=a]"sh[u^]n),  n.  [L.  libratio: 
  cf  F.  libration.] 
  1.  The  act  or  state  of  librating.  --Jer.  Taylor. 
  2.  (Astron.)  A  real  or  apparent  libratory  motion,  like  that 
  of  a  balance  before  coming  to  rest. 
  {Libration  of  the  moon},  any  one  of  those  small  periodical 
  changes  in  the  position  of  the  moon's  surface  relatively 
  to  the  earth,  in  consequence  of  which  narrow  portions  at 
  opposite  limbs  become  visible  or  invisible  alternately.  It 
  receives  different  names  according  to  the  manner  in  which 
  it  takes  place  as:  {(a)}  Libration  in  longitude,  that 
  which  depending  on  the  place  of  the  moon  in  its  elliptic 
  orbit,  causes  small  portions  near  the  eastern  and  western 
  borders  alternately  to  appear  and  disappear  each  month. 
  ({b})  Libration  in  latitude,  that  which  depends  on  the 
  varying  position  of  the  moon's  axis  in  respect  to  the 
  spectator,  causing  the  alternate  appearance  and 
  disappearance  of  either  pole.  ({c})  Diurnal  or  parallactic 
  libration,  that  which  brings  into  view  on  the  upper  limb, 
  at  rising  and  setting,  some  parts  not  in  the  average 
  visible  hemisphere. 

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