Co-altitude

Co-altitude is the complement of a body’s altitude, equal to 90 degrees minus the altitude, and it is identical to the zenith distance, the arc from the zenith to the body along the vertical circle. In the astronomical triangle it is the side joining the zenith to the body. Working with co-altitude rather than altitude lets the navigator state the triangle entirely in terms of polar distances and zenith distance, which the spherical cosine rule then solves for the computed altitude and azimuth. A small co-altitude means the body is high overhead; 90 degrees means it sits on the horizon.