Question 1589Navigation General

The correct answer is C) for inaccuracies in the reference level. When taking a sextant altitude measurement, the reading needs to be corrected to account for the fact that the sextant is measuring the angle between the celestial body and the visible horizon, not the true, geometric horizon. This difference is due to factors such as atmospheric refraction and the observer's height above the sea surface. Correcting the sextant altitude to the apparent altitude compensates for these inaccuracies in the reference level (the visible horizon) compared to the true, geometric horizon. The other options are incorrect because they do not fully address the need to correct for the difference between the visible and true horizons when determining the apparent altitude of a celestial body.