Marine Glossary

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Circle of Equal Altitude

Any circle of points on the earth's surface that is equal distance from the geographical position (GP) of a celestial body at a precise moment of time. All observers located anywhere on this circle would measure the same sextant height of the body at that moment. The center of the circle is the GP of the body, the radius of the circle is the zenith distance (z). The Ho they would observe is Ho = 90° - z. When taking the sights, the observers would all be looking toward the body in the center of the circle, which means some would be looking north, others, south or west, etc.. If the body were 60° high, the zenith distance would be 30°, which means the distance to the GP would be 30° x (60 miles/1°), or 1,800 miles. The LOPs used in celestial navigation are short segments of the circumference of this circle, which we approximate as straight lines.