A week or so after leaving Halifax, Nova Scotia, the convoy had scattered, there wasn't a ship in sight, therefore no station keeping and no string of flags from the Commodore, my presence on the bridge was no longer required. The sky was shrouded with cloud, the sea had an ominous look about it, especially so if you let your mind dwell on what may be lurking there and we hadn't seen the sun for days.
Our captain was one of those who, on occasions, liked to potter on deck and on this particular afternoon I observed him emerge from his cabin regaled in his workin gear. This consisted of an old bridge coat and a very faded tam-o-shanter. I was then seized upon and and told to assist him in checking the life boats. In the light of recent events anything connected with improving the lifeboats had my full approval.
Standing at a respectful distance by the starboard boat watching the Captain rummaging about within it, he must have forgotten I was there, consequently my attention span waned rapidly, only to be startled by the cry "There's the sun!" whereupon he leapt from the boat and raced to the bridge with the agility of half his age, in so doing, bringing me back to reality with a jolt. Sure enough there was the sun, briefly visible through a break in the cloud, in no time it was gone, glancing at the bridge I saw the Captain disappearing into the chartroom sextant in hand.
As a second year apprentice I had, as yet, only a cursory acquaintanceship with something called a P.Z.X. triangle and on reflection did not fully appreciate the significance of what I had just witnessed.
Before 1837 navigators determined their position by taking the altitude of the sun precisely at noon and by simple calculation worked out the latitude, then by comparing the local time with Greenwich time from their chronometers the longitude could be established, as is still done to this day. Unfortunately the fickleness of the weather, particularly in the higher northern and southern latitudes, would very often obscure the sun at the crucial moment, sometimes for days on end; and so it was with Captain Thomas Sumner on 17th Dec., 1837.
Leaving Charleston, South Carolina, and with strong favorable winds his ship was making a fast passage for Greenock on the Clyde. However, after passing the Azores the weather deteriorated and without a sight of the sun or stars for several days, with a dead reckoning position of some 40 miles from the Tusker Rock Light, soundings confirming he was on the continental shelf, a gale blowing and the weather very thick, plus a rocky Irish shore his position became a matter of some concern and what's more, it was night.
Being a wise young man of 30 years and the ship being 22 days out of Charleston, Captain Sumner turned her and sailed close-hauled off and on throughout the night and with the dawn still no sign of land. Back on a course just north of east and at about 10 o'clock there was a break in the cloud and he managed to get a single sight of the sun and the time of the observation from the chronometer. From this he was able to calculate a longitude by using the assumed latitude. This placed his position a little further eastward of his dead reckoning position. Still not satisfied, he assumed his latitude to be at fault and worked it all out again with a latitude 10 minutes in the north and then again 20 minutes to the north. He plotted these three positions on his chart and noticed they appeared to be in a straight line. Using a ruler to joint these positions and then extending the line towards the Bristol Channel it led directly to the Small's Rock Lighthouse which lies off the south west corner of the Welsh coast. It was then he made a simple but brilliant deduction - all the points, the ship, the three calculated positions and the Small's Rock Light at 10 o'clock must have had the same sun's altitude. Assuming the chronometer was keeping accurate time and if he altered course to sail along the line, he should, by his reasoning, find the lighthouse, this he did, and in less than an hour the Small's Rock Light was sighted almost dead ahead.
An important milestone had now be reached, Capt. Sumner had discovered the astronomical position line (Sumner Line), no longer had the navigator to rely on the noon sight for the ship's true position, in fact a single sight of the sun or stars at any hour would place the ship on a line. A second sight, even with a run in between, with the first transferred to cut the second line gives the observed position. It is a mystery how the mathematicians and the astronomers had not tumbled to this, the necessary means had been available for many centuries.
The position line is not actually a straight line, but part of the circumference of a huge circle with the geographical position of the sun or other body as the center of it.
In 1843 Captain Sumner published his discovery in A New and Accurate Method of Finding a Ship's Position At Sea, it proved to be a great success and was supplied to every ship in the U.S. Navy. Now comes the sad part, shortly after this his mind failed and in 1876 he died in a mental hospital at Taunton, Massachusetts, U.S.A., aged 69 years. It was now left to the French lieutenant Marcq St. Hilaire to simplify the method to its present form.