page_title: Thomas Hubbard Sumner
section_title: Discovery
hierarchical_section_title: Thomas Hubbard Sumner / Discovery
caption_attribution_description: English: A New and Accurate Method of Finding a Ship's Position at Sea, by Projection on Mercator's Chart, Capt. Thomas H. Sumner, July 1843, Thomas Groom &amp; Company of Boston
context_page_description: Thomas Hubbard Sumner was a sea captain during the 19th century. He is best known for developing the celestial navigation method known as the Sumner line or circle of equal altitude.
context_section_description: He discovered the (later so-called) line of position or circle of equal altitude, which he named "parallel of equal altitude" on a voyage from South Carolina to Scotland in 1837. On December 17, 1837, as he was nearing the coast of Wales, he was uncertain of his position after several days of cloudy weather and no sights.  A momentary opening in the clouds allowed him to take a sight of the sun which he reduced with his estimated latitude. Measuring the longitude depended on knowing the latitude accurately. Being uncertain about the latitude he reduced the sight again using 10' greater and 20' greater latitude, plotted the longitude for each one, and he observed that all three resulting positions were located in a straight line which also happened to pass through Smalls Lighthouse (off the coast of Pembrokeshire in Wales). He reasoned that he must be located somewhere along that line and so he set course along that line reasoning that he should eventually sight the light which, in fact he did. He realized that a single observation of the altitude of a celestial body determines the position of a line somewhere on which the observer is located. Sumner published his findings six years later in 1843 and this method of resolving a sight for two different latitudes and drawing a "line of position" through the two positions obtained was an important development in celestial navigation. The method was instantly recognized as important and a copy of the pamphlet describing the method was supplied to every ship in the United States Navy.
caption: A New and Accurate Method of Finding a Ship's Position at Sea, by Projection on Mercator's Chart, by Capt. Thomas H. Sumner, 1845
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