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Page Title: The Marine Sextant, Continued
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The  Marine  Sextant,  Continued Finding the Index  Error The  marine  sextant  will  measure  angles  accurately  if  it  is  properly adjusted  and  used  correctly.  The  senior  Quartermaster  is  responsible  for making  sure  that  any  adjustable  errors  are  properly  corrected  before  the sextant  is  used.  However,  practically  every  sextant  has  a  small  error called  index  error  (IC),  which  cannot  be  adjusted. Use  these  procedures  to  determine  index  error  every  time  you  use  a sextant  to  measure  angles.  An  index  error  correction  must  then  be applied  (added  or  subtracted)  to  every  angle  that  is  taken. Follow  these  steps  to  determine  index  error: Step Action 1. Hold  the  sextant  in  a  vertical  position  with  your  right  hand  on the handle. 2. Move  the  index  arm  to  approximately  zero  on  the  arc  scale with  your  left  hand  using  the  release  levers. 3. View  the  horizon  through  the  telescope. 4. Rotate  the  micrometer  drum  to  align  the  reflected  image  of  the horizon  with  the  direct  image. 5. Take a reading. 6. Repeat  steps  3  through  5  at  least  two  more  times. 7. Average  the  three  readings  to  determine  index  error. 8. Apply  the  index  error  correction  to  your  angle. IF  index  error THEN... is... zero no  correction  is  needed. positive subtract  the  amount  of  index  error. negative add  the  index  error. 8-28

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