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Page Title: Using the Sextant
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Using the Sextant General The  sextant  is  the  instrument  of  chief  importance  in  celestial  navigation. It  is  used  to  measure  the  altitude  of  a  heavenly  body  above  the  visible horizon.  Sextant  altitude  is  corrected  for  various  factors  to  determine the  body’s  true  (or  corrected)  altitude  above  the  celestial  horizon. Techniques Here  are  some  techniques  commonly  used  to  take  sights  with  the  marine sextant.  It  will  always  be  necessary  to  find  any  index  error  prior  to taking  sights;  refer  to  chapter  8  to  find  index  error. Use  the  following  step  action  table  for  the  general  steps  to  take  sights  on the  Sun.  The  steps  for  stars  and  planets  are  basically  the  same,  except you  would  omit  steps  2  and  4. Step Action 1. Hold  the  sextant  level  with  the  horizon  and  determine  index error. 2. CAUTION:  Set  shade  filters  in  place  now,  or  eye  burns may result. 3. Aim  the  sextant  to  a  point  on  the  horizion  directly  below  the Sun. 4. IF... THEN... the Sun is rising Move  the  index  arm  slowly  outward from  the  0°  position  until  the  Sun’s lower limb is just  below the horizon. the Sun is setting Move  the  index  arm  slowly  outward from the 0° position until the Sun’s lower limb is just  above  the horizon. 5. 6. Swing the arc. This  means  to  gently  move  your  hand  grasping the  sextant  handle  in  a  small  upward  arcing  motion.  Up  to  the left,  then  back  to  the  right.  You  will  see  the  reflected  image  of the  Sun  arc  back  and  forth. Give  the  recorder  a  standby  to  mark  (marking  the  exact  time  of the  sight).  Continue  swinging  the  arc  while  turning  the micrometer  drum  slightly  until  the  lower  limb  of  the  Sun touches  the  horizon.  At  that  exact  moment,  mark  the  time  of the  sight  and  record  the  sextant  altitude. 9-18

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