Question 777Navigation General

The correct answer is D) 90°. When the index and horizon mirrors of a properly adjusted sextant are at an angle of 45° to each other, the arc reads 90°. This is because the sextant measures the angle between the observed celestial body and the horizon, and when the mirrors are at a 45° angle, the full angle between them is doubled, resulting in a 90° reading on the sextant's arc. The other options are incorrect because 22 1/2° is half of 45°, 45° is the angle between the mirrors, and 60° is not the correct relationship between the mirror angle and the sextant reading.