easy calculations and adjustments 65
HOW TO CHECK THE INDEX ERROR
Index error is caused by minor alignment errors in the mirror. This causes a
sextant to read too high or too low. If you fail to correct the index error, your
position could be off by several miles. It’s simple to measure and apply index
error. Always find index error before using any marine sextant.
Q Set the mark on the index arm and micrometer drum to zero (as shown
in the fi rst photo).
Q Look through the scope and sight the horizon or a distant horizontal
surface. Crank the micrometer drum slowly up and down. Watch the
image split, then bring it together to form a straight line.
Q Now read the micrometer drum as described in the second and third
photos. How many minutes is it on or off the arc? Check it once or
twice more. You should get similar readings.To correct a sextant reading with minutes on the arc, you subtract the index
error from the observed angle. To correct a sextant reading with minutes off
the arc, you add the index error to the observed angle.
Turn the micrometer drum toward you about a quarter turn. You’ll notice the minutes
decrease. When adjusting or checking for error, this is called off the arc. Add the minutes
to correct an observation.