Question 821Navigation General

The correct answer is D) the horizon glass not being parallel to the index mirror. The index error of a sextant is primarily caused by the horizon glass not being parallel to the index mirror. This is because the index error is the angle between the index mirror and the horizon glass, which needs to be precisely 90 degrees for accurate celestial observations. If the horizon glass is not parallel to the index mirror, it will introduce an error in the sextant's measurements. The other options are incorrect because they do not directly address the primary cause of the index error. Human error, improper correction of other sextant errors, and the horizon glass not being parallel to the horizon mirror can all contribute to inaccurate celestial observations, but they are not the primary source of the index error.