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The Marine Sextant, Continued
Finding the
Index Error
The marine sextant will measure angles accurately if it is properly
adjusted and used correctly. The senior Quartermaster is responsible for
making sure that any adjustable errors are properly corrected before the
sextant is used. However, practically every sextant has a small error
called index error (IC), which cannot be adjusted.
Use these procedures to determine index error every time you use a
sextant to measure angles. An index error correction must then be
applied (added or subtracted) to every angle that is taken.
Follow these steps to determine index error:
Step
Action
1.
Hold the sextant in a vertical position with your right hand on
the handle.
2.
Move the index arm to approximately zero on the arc scale
with your left hand using the release levers.
3.
View the horizon through the telescope.
4.
Rotate the micrometer drum to align the reflected image of the
horizon with the direct image.
5.
Take a reading.
6.
Repeat steps 3 through 5 at least two more times.
7.
Average the three readings to determine index error.
8.
Apply the index error correction to your angle.
IF index error
THEN...
is...
zero
no correction is needed.
positive
subtract the amount of index error.
negative
add the index error.
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