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Altitude  Corrections,  Continued - 14221_278
Altitude  Corrections,  Continued Corrections Defined Name Index  Error Refraction We  will  explore  each  altitude  correction  in  detail.  Applying  altitude corrections  is  the  starting  point  for  reducing  sights  for  any  observation. Description The  amount  of  instrument  error  in  the  sextant  (covered  in  chapter  8). Earth  is  wrapped  in  a  blanket  of  atmosphere  more  than  50  miles  deep. Density  of  the  atmosphere,  like  that  of  the  ocean,  increases  with  depth and  is  greatest  at  the  bottom,  next  to  Earth’s  surface.  Light  rays  do  not follow  a  straight  line  when  passing  through  atmosphere  of  different densities,  but  are  slightly  bent  into  a  gentle  arc.  This  phenomenon  is called  refraction.  Refraction  is  defined  as  the  deviation  of  light  rays from  a  straight  line  caused  by  their  passage  obliquely  through  mediums of  different  density.  The  measure  of  refraction  is  the  angular  difference between  the  apparent  rays  of  light  from  an  observed  celestial  body  and its true direction. Dip The  effect  of  refraction  is  always  to  make  the  observed  altitude  greater than  the  true  altitude.  Consequently,  refraction  correction  is  always subtracted  from  the  sextant  altitude.  Since  refraction  is  caused  by  the oblique  passage  of  rays  through  the  atmosphere,  rays  from  a  body  in  the observer’s  zenith,  intersecting  the  atmosphere  at  right  angles,  are  not refracted.  Maximum  refraction  occurs  when  a  body  is  on  the  horizon, amounting  then  to  between  34  and  39  minutes  of  arc.  The  amount  of refractions  depends  on  atmospheric  conditions.  Density  of  the atmosphere  varies  with  barometric  pressure  and  temperature.  Refraction varies  with  density  and  also  with  the  body’s  altitude.  Because  refraction varies  with  atmospheric  conditions,  and  the  effect  of  atmospheric  con- ditions  at  low  altitudes  cannot  be  estimated  with  complete  accuracy, observations  of  bodies  below  10°  should  be  regarded  with  suspicion. Refraction  has  no  effect  on  the  azimuth  of  a  celestial  body  because  it takes  place  entirely  in  the  vertical  plane  of  passage  of  the  light  rays. The  higher  an  observer’s  position  is  above  the  surface  of  the  Earth,  the more  he/she  must  lower  (or  dip)  the  line  of  vision  to  see  the  horizon. Logically,  then,  all  altitude  observations  must  be  corrected  for  the  height of  eye.  Refer  again  to  figure  9-9,  and  you  will  see  why  a  dip correction  is  always  subtracted. Failure  to  correct  for  dip  from  a  height  of  10  feet  will  result  in  an  error of  3  miles  in  a  line  of  position.  From  the  bridge  of  the  average destroyer,  the  resulting  error  would  be  approximately  10  miles. 9-21

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