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The Celestial LOP - 14221_273
The Celestial LOP General Information You  have  seen  how  lines  of  position,  obtained  through  bearings  on terrestrial  objects,  are  used  to  fix  a  ship’s  position  in  piloting.  You know  that  a  line  of  position  (LOP)  is  a  locus  of  possible  positions  of  the ship.  In  other  words,  the  ship’s  position  must  be  somewhere  along  that line.  A  fix,  by  definition,  is  a  relatively  accurate  determination  of latitude  and  longitude.  In  practice,  this  position  is  the  intersection  of two  or  more  lines  of  position;  but  often  it  is  not  the  ship’s  exact  position because  you  can  always  assume  some  errors  in  observation,  plotting,  and the like. The  celestial  navigator  must  establish  lines  of  position  by  applying  the results  of  observations  of  heavenly  bodies.  A  line  of  position  obtained at  one  time  may  be  used  at  a  later  time.  All  you  need  to  do  is  move  the line  parallel  to  itself,  a  distance  equal  to  the  run  of  the  ship  in  the interim,  and  in  the  same  direction  as  the  run.  Such  a  line  of  position cannot  be  as  accurate  as  a  new  line  because  the  amount  and  direction  of its  movement  can  be  determined  only  by  the  usual  DR  methods.  If  two new  lines  cannot  be  obtained,  however,  an  old  line,  advanced  and intersected  with  a  new  one,  may  be  the  only  possible  way  of  establishing a  fix.  Naturally,  the  distance  an  old  line  may  be  advanced  without  a substantial  loss  of  accuracy  depends  on  how  closely  the  run  can  be reckoned. In  celestial  navigation,  as  in  piloting,  you  essentially  are  trying  to establish  the  intersection  of  two  or  more  lines  of  position.  A  single observation  and  the  resulting  LOP  is  insufficient  to  obtain  a  fix. The  most  accurate  method  of  obtaining  a  celestial  fix  is  to  take  sights  on many  bodies  in  a  short  time.  For  example,  it  is  quite  common  to  take sights  on  six  or  more  stars  in  a  period  of  15  minutes  or  less.  Taking sights  on  many  bodies  allows  the  observer  to  identify  and  throw  out LOPS  with  obvious  errors. Determining the You  might  be  entitled  to  complain  that  much  has  been  said  concerning LOP what  an  LOP  tells  you,  but  very  little  has  been  told  about  how  you determine  it  in  the  first  place.  We  are  coming  to  that  part  now. The  first  item  is  to  take  on  a  heavenly  body  or  bodies  and  then  reduce the  sights.  Reducing  the  sights  taken  gives  you  the  information  you need  to  plot  the  LOP.  The  LOPS  then  gives  you  the  resulting  fix. 9-16

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