Sumner position line

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Effectively the concept on which nearly all modern celestial navigation is based. The voyage which gave birth to the idea of the astronomical position line was made in December 1837 when an American, Captain Thomas H. Sumner, was approaching the south coast of Ireland on passage from Charleston, SC, to Greenock in Scotland. The weather had been overcast and foggy for some days, and Sumner, unable to get a sight for days, was navigating by dead reckoning. Of 17 December, Sumner wrote: ‘the ship was kept on ENE under short sail with gales. At about 10 a.m. an altitude of the sun was observed and the chronometer time noted; but … it was plain that the latitude by DR could not be relied upon. Using this latitude in finding the longitude by chronometer it was found to put the ship 15° of longitude east … which is 9° nautical miles. This seemed to agree … with the DR … but the observation was tried with a latitude 10 further north. This placed the ship ENE 27 miles of the former position and was tried again with a latitude 20° north of the DR. This also placed the ship still further ENE and still 27 miles. These three positions were then seen to lie in the direction of the Smalls Light. It appeared that the observed altitudes must have happened at all three points and at the Smalls Light and at the ship at the same time, and it followed that the Smalls Light must bear ENE,’ which indeed proved to be the case. Although he (rightly) described the concept in terms of position circles the first astronomical position line was nevertheless undoubtedly that through the Smalls Light. Sumner described his method in a pamphlet published in Boston in 1843.

See also marcq saint-hilaire method.

See also marcq saint-hilaire method.

Mike Richey

Sumner position line, 17 December 1837

Subjects: Maritime History.

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