1837 - A Fortunate Incident of Navigation

Captain Thomas Hubbard Sumner, 30 years old, commanding the sailing ship Cabot, travelling from Charleston, Virginia, to Greenock, Scotland, makes a practical discovery.

Due to constant bad weather all along his journey, he has navigated on a dead reckoning basis until he finally encounters a piece of blue sky which gives him sight of the sun. He uses the altitude of the sun to make a calculation of longitude based on his estimated latitude. But, quite uncertain of his latitude, he repeats calculations with different possible latitudes and discovers that the calculated positions are all on a same line. He correctly infers from the alignment that, wherever the real position may be, it necessarily lies somewhere on said line. Thus he decides to follow it as a bearing to a landmark which the map shows, the Smalls lighthouse. He effectively happens to land there – at 16 miles west of the Skomer island in Wales, 260 miles south of Greenock. His trick has been helpful.

¤ It is worth noticing the impact of bad weather on dead reckoning navigation : landing South of Wales instead of Scotland without any fault in navigation.

In terms of astronomy, there is nothing extraordinary about Sumner's find. What he has brought out is the large circle of the positions of equal altitude of the sun above the horizon – a circle centred on the subsolar point, which is the projection of the sun to the surface of the earth, or, if one prefers, the position on the surface of the earth where the sun is currently at the azimuth. This circle of equal altitude is so large that an arc is very close to its tangent so looks like a straight line.

The first useful information for a mariner is this unheard-of possibility of drawing a line of potential positions from a single shooting. For an astronomer, quietly seated at his observatory, the existence of the great circle of equal altitude is trivial and of little interest ; for a mariner lost at sea, this may turn to be a lifeline.

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NOTE IN RETROSPECT FROM TODAY

NOTE - About the aftermath.

Sumner will think about it at length to the point of inventing a method of celestial navigation.