An error, known as the index error, is introduced if there is a small lack of parallelism of the horizon glass. Index correction is resolved by the following procedure.
Set the sextant near zero. Hold the sextant vertically and sight toward the horizon. Use the micrometer drum to bring the direct and reflected horizon exactly in line. If the sextant reading is zero, there is no error. If the reading is not zero, the amount of error is the index correction. If the index mark is to the left of the zero on the arc of the limb, then the reading is too large, and this index correction must be subtracted from the sextant altitude. If the index mark is to the right of zero (off the arc), the reading is too low, and this amount must be added to the sextant altitude.
Set the sextant near zero. Hold the sextant vertically and sight toward the horizon. Use the micrometer drum to bring the direct and reflected horizon exactly in line. If the sextant reading is zero, there is no error. If the reading is not zero, the amount of error is the index correction. If the index mark is to the left of the zero on the arc of the limb, then the reading is too large, and this index correction must be subtracted from the sextant altitude. If the index mark is to the right of zero (off the arc), the reading is too low, and this amount must be added to the sextant altitude.
Remember:
If it's on (the arc), it's off; if it's off (the arc), it's on. The amount of index correction is obtained as follows: If the index mark is on the arc, the sextant is read in the usual way. The reading is the index correction to be subtracted. Always read the mark that is on the arc of the limb to the right of the index mark.
If it's on (the arc), it's off; if it's off (the arc), it's on. The amount of index correction is obtained as follows: If the index mark is on the arc, the sextant is read in the usual way. The reading is the index correction to be subtracted. Always read the mark that is on the arc of the limb to the right of the index mark.
