Determining the Ship’s Position Using Horizontal Sextant Angles

 

Horizontal  sextant  angles  give fixes of great accuracy that are  not affected  by  any  error  of  the  compass.  A  fix  by  horizontal  sextant  angles is  labeled  the  same  as  a  visual  fix  with  a  small  circle  around  the  position and  the  time  of  the  fix  close  to  the  fix  symbol.

 

Horizontal  sextant  angles  used  in  conjunction  with  a  computer-assisted positioning  program  are  the  most  common  method  used  by  the  Coast Guard  to  position  aids  to  navigation.

 

Horizontal  sextant  angles  should  be  taken  as  nearly  simultaneously  as possible,  preferably  by  two  people  on  a  predetermined  signal.  However, one  person  can  obtain  both  angles  if  the  ship  is  not  moving  quickly.

 

To  obtain  a  fix  using  sextant  angles,  you  must  have  three  fixed visual  objects,  and  those  objects  must  be  identifiable  on  the  chart.

 

Follow  these  steps  to  obtain  horizontal  sextant  angles:

 

1.  Hold  the  sextant  horizontally  with  your  right  hand.

 

2.  View  the  left  object  directly  through  the  telescope.

 

3.  Release  the  index  arm  with  your  left  hand  on  the  release levers  and

     swing  the  arm  so  that  the  index  mirror  reflects  the center  object  in  the

     horizon  glass  below  the  left  object.

4.  Rotate  the  micrometer  drum  to  fine  adjust  the  reflected object  in  line

     with  the  object  viewed  directly.

 

5. Take  a  reading  of  the  angle.

 

1. Do  you  have  two  people  taking  angles?

 

Ø      If  yes,  both  angles  are  ready to  be  plotted.

Ø      If  no,  repeat  steps  1  through  5  using  the  center  object

                   viewed directly  and  the  right  object  viewed  reflected.

 

 

Now, select three objects which are on your chart and in sight.  Measure the angle on each side of the middle object (which is common to both angles).

See illustration below…

 

 

Taking angular measurement with Sextant

 

Using the Three-Arm Protractor

 

The  two  angles  measured  are  plotted  using  a  three-arm  protractor.  This instrument,  made  of  brass  or  plastic,  consists  of  a  circular  scale  that  can be  read  to  fractions  of  a  degree  or  minutes  of  arc,  and  to  which  the  three arms  are  attached.  The  center,  or  index  arm,  is  fixed  and  the  zero graduation  of  the  protractor  coincides  with  the  straightedge  of  this  arm. The  other  arms  are  movable  and  can  be  set  and  locked  at  any  angle relative  to  the  fixed  arm.

 

The center arm is placed on the center object of your sights, with corresponding arms on the object to the left and the right set at ANGLES read from sextant.

The pivot point (center of protractor) is the resulting position at the time of the sights.

If  the  three  objects  and  the  ship  all  lie  on  the  circumference  of  a  circle, the  fix  is  NOT  reliable.  When  this  happens,  it  is  called  a  swinger  or revolver  and  your  ship  could  be  anywhere  along  the  circle  and  still  have the  same  two  angles  to  the  three  objects.

Procedure for Plotting the Fix

1. Place  the  three-arm  protractor  on  the  chart.

 

2. Position  the  center  (fixed)  index  line  so  that  it  passes  through the  center  object.

 

3. Move  the  protractor  slowly  across  the  chart  until  all  three  arms are  aligned  with  the  three  objects.

 

4. Mark  the  ship’s  position  on  the  chart  by  inserting  a  pencil point  in  the  center  of  the  protractor  (pivot  point).

 

5. Check  to  see  that  the  fix  is  not  a  swinger  or  revolver.

 

This method using a sextant for Coastal & Inland navigation can be more accurate than GPS!