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Visibility of Lights, Continued - 14221_108
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Luminous Range Diagram - 14221_110
How to Compute the Visibility of a Light Information Frequently,  the  navigator  will  want  to  know  at  what  time  and  position on  the  ship’s  track  a  given  light  might  be  sighted.  This  information  is especially  important  when  the  ship  is  making  a  landfall.  Failure  to  sight certain  lights  when  expected  could  mean  that  a  navigational  error  has been  made.  The  distance  calculated  is  termed  the  computed  visibility  of the light. Rule:  When  you  compute  the  visibility  of  a  light,  the  computed visibility  will  NEVER  exceed  the  light’s  luminous  range. Examples The  following  examples  illustrate  the  recommended  procedure  for determining  the  visibility  of  a  light.  Bear  in  mind  that  computed visibility  cannot  be  greater  than  the  luminous  range. Example  1:  Determine  the  visibility  of  light  Alpha  for  an  observer  with a  height  of  eye  of  50  feet. Solution:  From  the  Light  List,  the  nominal  range  is  determined  to  be  20 miles;  the  height  of  light  Alpha  is  90  feet  above  the  water.  Determine horizon  distance  from  table  4-1. Height  of  eye  for  50  feet 8.3 miles Height  of  light  (80  feet) 11.1  miles Computed  visibility 19.4 miles Nominal  range 20.0  miles Answer: 19.4 miles Example  2:  Determine  the  visibility  of  light  Bravo  for  an  observer  with a  height  of  eye  of  35  feet. Solution:  From  the  Light  List,  determine  the  nominal  range  (10  miles) and  the  height  of  the  light  above  water  (80  feet).  Determine  horizon distance  from  table  4-1. Height  of  eye  for  35  feet Height  of  light  (80  feet) Computed  visibility Nominal  range Answer: 6.9 miles 10.5 miles 17.4 miles 10.0 miles 10.0 miles 4-11

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