What does Co-Altitude refer to in navigation?

Co-Altitude in navigation specifically refers to the distance between the assumed position of a vessel and its actual geographic position. This concept is critical in celestial navigation, where navigators calculate their position based on observations of celestial bodies. The assumed position is typically derived from prior calculations or estimations, while the geographic position is determined using celestial sights. Understanding Co-Altitude allows navigators to measure how far off they are from their intended course or location, enabling them to make the necessary adjustments to their navigation plan.

While the other options involve relevant terms in navigation, they do not accurately describe the meaning of Co-Altitude. For instance, the angular distance to a celestial object is more about the observer's line of sight rather than positional accuracy in navigation. The altitude of a vessel is a separate concern focused solely on the vessel's height above sea level, while the difference in altitude between celestial bodies pertains to their positions in the sky, rather than their relevance to the vessel's navigation.