Circle of equal altitude

The circle of equal altitude, also called circle of position (CoP), is defined as the locus of points on Earth on which an observer sees a celestial object such as the sun or a star, at a given time, with the same observed altitude.

It was discovered by the American sea-captain Thomas Hubbard Sumner in 1837, published in 1843 and is the basis of an important method in celestial navigation.

Sumner discovered the line on a voyage from South Carolina to Greenock in Scotland in 1837.

On December 17, as he was nearing the coast of Wales, he was uncertain of his position after several days of cloudy weather and no sights.

The method was quickly recognized as an important development in celestial navigation, and was made available to every ship in the United States Navy.