Question 803Navigation General

The correct answer is C) index mirror. The index mirror is the part of the sextant that is mounted directly over the pivot of the index arm. This is a key component of the sextant, as it reflects the celestial body being observed, allowing the user to align it with the visible horizon to take a reading. The other answer choices are incorrect because: A) the micrometer drum is used to measure the angle between the celestial body and the horizon, B) the horizon glass allows the user to view both the celestial body and the visible horizon, and D) the telescope is used to magnify the celestial body being observed, but is not directly mounted over the index arm pivot.