66 seamanship secrets
Example: You check the index error on your sextant and determine that it has
2 minutes of error on the arc. You observe an angle between two objects of
24 degrees 12 minutes. Subtract 2 minutes to correct the reading to 24 degrees
10 minutes.
Example: You check the index error on your sextant and determine that it
has 3 minutes of error off the arc. You observe an angle between two objects
of 98 degrees 54 minutes. Add 3 minutes to correct the reading to 98 degrees
57 minutes.THREE STEPS TO ADJUST YOUR SEXTANT
If the error is more than 2 minutes on
or off the arc, you’ll need to go through
the adjustment process shown below.
Follow each step consecutively to re-
duce or eliminate the index error.
First, check the type of screws on the
back of the index mirror and horizon glass. Read the sextant manufacturer’s
recommendation for adjustment tools.
- Adjust the index mirror perpendicular to the frame. Remove the
telescope. Set the micrometer drum to zero. Set the index arm to
35 degrees. Turn the sextant around so that you’re looking at the index
mirror. Hold the sextant up to your eye and look directly into the mirror.
Now, rotate the sextant slowly to the left until you see the actual and
refl ected image of the arc. If you see a broken line, fi nd the screw on the
back of the index mirror. You may need a jeweler’s screwdriver or Allen
wrench. Insert the tool into the screw and hold it. Bring the sextant back
up to your eye and once again rotate it to get the actual and refl ected
image of the arc in the mirror. Turn the screw until you have a straight
line. Th en go to Step 2. - Remove side error. Insert the telescope. Set the micrometer drum and index
arm to zero. Find a distant vertical object such as a fl agpole or water tank.
Sight the object through the horizon glass. Now, keeping the sextant vertical,
move it slightly to the left and right. You’ll see two images, the object itself
and its refl ected image. As you move the sextant from side to side, they
should stay lined up on top of one another. If not, you need to adjust the
horizon glass.
The horizon glass has two screws. Use the horizon glass screw
farthest from the frame. Insert your tool in the screw and then sight
the vertical object again. Adjust the screw incrementally up or down
to vertically align the actual and reflected object. Check the adjustment
again when you are done. Then go to Step 3.
Tools Needed to Adjust
Your Sextant
X jeweler’s screwdriver or hex-key
wrench set