Question 834Navigation General

The correct answer is A) micrometer drum. The index error of a sextant is determined by adjusting the micrometer drum. The index error is the difference between the sextant's reading and the true angle of the observed celestial body. To correct for this error, the sextant's index mirror is adjusted by turning the micrometer drum until the direct and reflected images of the celestial body are aligned, indicating a zero reading. This adjustment to the micrometer drum is the index error and must be applied to all sextant observations to obtain the true angle. The other options are incorrect because the horizon glass and index mirror are not adjusted to determine the index error, and the sextant frame does not contain the components necessary to make this adjustment.