If you've visited Ships, Clocks & Stars (open until 4 January 2015), you probably noticed the rather fine portrait of the young Edmond Halley (1656-1742). Halley is now mostly known for correctly predicting the return of 'his' comet, but he was active in many fields and interested in longitude and navigation throughout his long life, making him a fitting subject for this week's Longitude Legends.

Edmond Halley, by Thomas Murray, about 1690 - The Royal Society Edmond Halley, by Thomas Murray, about 1690 © The Royal Society

At the age of 18, Halley was in Greenwich with John Flamsteed and Robert Hooke when they met to discuss the proposed site for the new observatory that Flamsteed would occupy as the first Astronomer Royal, a position created in 1675 in part for the purpose of improving knowledge of longitude at sea. In 1714 Halley was appointed as a Commissioner of Longitude through his post as Savilian Professor of Geometry, and later also as the Astronomer Royal. And in the late 1720s, it was Halley whom John Harrison approached with his plans for a marine timekeeper, and who directed Harrison to George Graham, the leading clockmaker who helped Harrison to bring those plans to fruition.

Like many scientifically-minded men of the period, Halley was also actively involved in the search for a method of finding longitude at sea, but where most were content to pursue the matter from behind their desk or telescope, Halley was of a more adventurous disposition and ready to try his theories at sea.

Now regular readers of this blog will know that there were two principal contenders for finding longitude at sea, lunar-distance and timekeeper, but there were several other early proposals that merited serious consideration, and one of these was magnetic variation. Magnetic variation (or declination) is the angle by which a compass varies east or west from true north, a variation that itself varies over time (known as secular variation). It was thought that if a pattern could be found in this variation, it might provide a way of finding longitude and Halley resolved to go to sea to collect the data to test the idea.

In 1693 Halley and Benjamin Middleton (another Royal Society fellow) requested the Society's support to petition the government to co-fund a voyage to measure the magnetic variation. They proposed that if the navy would give them a vessel, Middleton would pay for the crew and victuals, while Halley would undertake the scientific observations - and their intention at this stage was nothing less than to sail the whole globe.

The government approved the petition and the Admiralty ordered that a ship be built for the voyage. The ship was ready by April 1694 and named Paramour, although Halley habitually used the spelling Paramore in his correspondence and ship's logs.

For a variety of reasons - not all of them known - it was not until October 1698 that Paramore set sail, now with the modified aim of sailing only round the Atlantic. Middleton had disappeared from the project by then, and it was now being funded entirely by the government; Halley himself was commissioned the ship's master and commander, despite having only limited maritime experience as a passenger and coastal surveyor.

Halley's instructions - derived from his own proposals - were to measure the compass variations, to ascertain the longitude and latitude of the places he visited, and to seek the Terra Incognita that was thought to lie in the southern Atlantic. But alas his first voyage did not go well, owing to the recalcitrant behaviour of some of his officers, who were animated against him by his lieutenant, Edward Harrison. Unbeknown to Halley, Harrison held a grudge against him for having given a dismissive review of Harrison's book, Idea Longitudinis, to the Admiralty, and Halley was at a loss to understand the 'intollerable usage' he received from him. He only learned of the reason after his early return to England to court martial Harrison, who was found not guilty of disobedience to command and given merely a severe reprimand, greatly to Halley's displeasure.

Three months later Halley was back at sea in the Paramore, with similar instructions but a new and amenable crew, and this time Halley completed his voyage, sailing in a figure of eight around the Atlantic.

Along the way he saw creatures that have defied modern identification, was mistaken for a pirate and shot at, got arrested in Pernambuco by a rogue consul, contracted a dangerous tropical disease at 'Barbadoes', nearly lost ship and lives when enveloped by icebergs in heavy fog, populated the island of Trindade with 'Goats and Hoggs', and planted a flag there in the name of King William - an action that prompted a minor diplomatic incident between Brazil and Britain some 200 years later.

Halley arrived back in England in September 1700 and immediately began preparing a chart of his magnetic data, which he published the following spring. The chart was a handsome product and the first published use of isogonic lines to join points of equal magnetic variation (known into the nineteenth century as Halleyan lines).

But was the chart successful in helping find one's longitude at sea? Well, the map is said to have been an improvement on any that had gone before it, but if Halley's own longitude at sea was not accurately known, then his locations and data were necessarily defective. Two such examples are that Halley depicted Cape Horn approximately 10° further west than we now know it to be, and the longitude he recorded when he was east of the island of St Helena, is actually west of that island, and so the associated isoline might well lead a navigator astray.

However, the chart was generally well-received and Halley was paid a bonus by the Lords of the Admiralty. He didn't subsequently update the chart to account for secular variation, but others did, suggesting it was considered a worthwhile and useful endeavour. Halley did produce another work deriving from his Atlantic chart, in which he had asked that fellow navigators supply him with magnetic data from other oceans, and in 1702 he produced an extended chart incorporating that data. This second, splendid chart covered much of the world - and is now a part of the National Maritime Museum's permanent collections and currently prominently displayed in Ships, Clocks & Stars.

A new and correct sea chart of the whole world shewing the variations of the compass (NMM G201:1/1) A new and correct sea chart of the whole world shewing the variations of the compass (NMM G201:1/1)
A new and correct sea chart of the whole world shewing the variations of the compass (NMM G201:1/1A) A new and correct sea chart of the whole world shewing the variations of the compass (NMM G201:1/1A)

Kate Morant is studying for an MA in Early Modern History at Birkbeck, University of London and blogs about Halley's voyages at Halley's Log; she can be found on twitter @KateMorant. Edmond Halley is also on twitter @HalleysLog, where he has just finished 'live' tweeting the logs of his two Atlantic voyages. He is now back in London but will sail again on a third voyage to survey the English Channel in June 1701 (2015).