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Altitude  Corrections,  Continued - 14221_279
Altitude  Corrections,  Continued Name Parallax Description Parallax  is  the  difference  between  the  altitude  of  a  body,  as  measured from  Earth’s  center,  and  its  altitude  (corrected  for  refraction  and  dip)  as measured  from  Earth’s  surface.  Altitude  from  the  center  of  Earth  is bound  to  be  greater  than  from  the  surface.  Consequently  parallax  is always  a  plus  correction. Parallax  increases  from  0°  for  a  body  directly  overhead  to  a  maximum for  a  body  on  the  horizon.  In  the  latter  instance,  it  is  called  horizontal parallax  (HP).  Parallax  of  the  Moon  is  both  extreme  and  varied because  of  its  changing  distance  from  Earth  in  its  passage  through  its orbit.  Parallax  of  the  Sun  is  small;  parallax  of  the  planets  is  even smaller.  For  the  stars,  parallax  is  so  tiny  it  is  negligible. Semidiameter The  true  altitude  of  a  body  is  measured  to  the  center  of  that  body. Because  the  Sun  and  Moon  are  of  appreciable  size,  the  usual  practice  is to  observe  the  lower  limb.  Therefore,  semidiameter  correction  must  be added.  It  follows,  then,  that  if  the  upper  limb  of  either  body  is observed,   the   semidiameter   correction   is   subtractive.   Semidiameter correction  amounts  to  about  16  minutes  of  arc  for  either  the  Sun  or Moon.  Stars  are  considered  as  points,  and  they  require  no  semidiameter correction.  When  observing  a  planet,  the  center  of  the  planet  is  visually estimated  by  the  observer,  so  there  is  never  a  semidiameter  correction. Remarks In  concluding  the  subject  of  altitude  corrections,  remember  that  some tables  for  altitude  corrections  (the  Nautical  Almanac,  for  example) combine  two  or  more  of  the  corrections  for  refraction,  parallax,  and semidiameter. The  correction  for  height  of  eye  (dip)  appears  in  a  separate  table  for  use with  all  bodies.  Index  error,  which  is  impossible  to  include  in  such tables,  should  always  be  determined,  recorded,  marked  plus  or  minus, and  applied  before  any  of  the  tabulated  corrections. 9-22

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