Zenith distance

Zenith distance is the arc from the observer’s zenith to a body along the vertical circle, equal to 90 degrees minus the altitude, and identical to the co-altitude. It is the side of the astronomical triangle joining the zenith to the body. In a meridian-altitude sight, latitude follows directly from zenith distance and declination: latitude equals zenith distance combined with declination, added or subtracted by name. A small zenith distance means the body is nearly overhead; 90 degrees places it on the horizon. The geographical position of the body lies at a distance from the observer in nautical miles equal to the zenith distance in arcminutes.