Lorsque l'épave du navire Endurance d'Ernest Shackleton a été retrouvée à près de 10 000 pieds sous la surface de la mer de Weddell en Antarctique en mars 2022, elle était située à seulement 4 milles de sa dernière position connue, comme l'a enregistré le capitaine et navigateur de l'Endurance, Frank Worsley, en novembre. 1915.

C'est un degré de précision étonnant pour une position déterminée avec des outils mécaniques, des tableaux de numéros de référence d'une longueur de livre, un stylo et du papier.

L'expédition à la recherche du navire avait recherché une zone sous-marine de 150 miles carrés - un cercle de 14 miles de diamètre. Personne ne savait à quel point le calcul de la position de Worsley avait été précis, ni quelle distance le navire aurait pu parcourir en coulant.

Mais en tant qu'historien de l'exploration de l'Antarctique, je n'ai pas été surpris de découvrir à quel point Worsley était précis, et j'imagine que ceux qui cherchaient l'épave ne l'étaient pas non plus.

La navigation était essentielle

L'Endurance avait quitté l'Angleterre en août 1914, l'Irlandais Shackleton espérant devenir le premier à traverser le continent Antarctique de part en part.

Mais ils n'ont même jamais atterri en Antarctique. Le navire s'est retrouvé coincé dans la glace de mer dans la mer de Weddell en janvier 1915, forçant les hommes à descendre du navire dans des tentes dressées sur l'océan gelé à proximité. La force de la glace écrasa lentement l'Endurance, le coula 10 mois plus tard et lança ce qui allait devenir une incroyable - et presque incroyable - saga de survie et de navigation par Shackleton et son équipage.

Le propre leadership de Shackleton est devenu une légende, tout comme son engagement à s'assurer qu'aucun homme ne soit perdu du groupe sous son commandement, bien que trois membres du groupe de 10 hommes de l'expédition dans la mer de Ross aient péri.

Moins connue est l'importance des compétences en navigation de Worsley, 42 ans, un Néo-Zélandais qui a passé des décennies dans la marine marchande britannique et la réserve de la Royal Navy. Sans lui, l'histoire de la survie de Shackleton aurait probablement été très différente.

Marquer le temps

La navigation nécessite de déterminer l'emplacement d'un navire en latitude et en longitude. La latitude est facile à trouver à partir de l'angle du Soleil au-dessus de l'horizon à midi.

La longitude nécessitait de comparer le midi local - le moment où le Soleil était à son point culminant - avec l'heure réelle à un autre endroit où la longitude était déjà connue. La clé était de s'assurer que la mesure du temps pour cet autre endroit était exacte.

Faire ces observations astronomiques et faire les calculs qui en résultent était déjà assez difficile sur terre. Sur l'océan, avec peu de points terrestres fixes visibles, par mauvais temps, c'était presque impossible.

La navigation dépendait donc largement de "l'estime". Il s'agissait du processus de calcul de la position d'un navire en utilisant une position préalablement déterminée et en incorporant des estimations de la vitesse et de la direction du navire. Worsley l'appelait "le calcul des parcours et de la distance par le marin".

Viser la terre

When the Endurance was crushed, the crew had to get themselves to safety, or die on an ice floe adrift somewhere in the Southern Ocean. In April 1916, six months after the Endurance sank, the sea ice on which they had camped began to break up. The 28 men and their remaining gear and supplies loaded into three lifeboats—the James Caird, Dudley Docker and Stancomb Wills—each named for major donors to the expedition.

Worsley was in charge of getting them to land. As the journey began, Shackleton "saw Worsley, as navigating officer, balancing himself on the gunwale of the Dudley Docker with his arm around the mast, ready to snap the sun. He got his observation and we waited eagerly while he worked out the sight."

To do that, he compared his measurement with the time on his chronometer and written tables of calculations.

And so began one of the most remarkable rescue missions in history. In a small open lifeboat, six men sailed 800 miles across some of the roughest seas on the planet to get help. We think they navigated with these instruments, on display in #PolarWorlds pic.twitter.com/4lxgzfR1BL

— Dr. Claire Warrior (@ClaireWarrior1) March 9, 2022

A last hope of survival

Once they managed to arrive on a little rocky strip called Elephant Island, off the coast of the Antarctic Peninsula, they still faced starvation. Shackleton believed that the only hope of survival lay in fetching help from elsewhere.

Worsley was ready. Before the Endurance was crushed, he had "worked out the courses and distances from the South Orkneys to South Georgia, the Falklands and Cape Horn, respectively, and from Elephant Island to the same places," he recalled in his memoir.

The men used parts of the other lifeboats to reinforce the James Caird for a long sea journey. Every day, Worsley "watched closely for the sun or stars to appear, to correct my chronometer, on the accuracy of which our lives and the success of the journey would depend."

On April 24, 1916, Worsley got "The first sunny day with a clear enough horizon to get a sight for rating my chronometer." That same day, he, Shackleton and four other men set off under sail in the 22.5-foot James Caird, carrying Worsley's chronometer, navigational books and two sextants, used for fixing the position of the Sun and stars.

The boat journey

These men, in this tiny boat, were going from one pinpoint of rock in the Southern Ocean to another, facing high winds, massive currents and choppy waters that could push them wildly astray or even sink them. The success of this voyage depended on Worsley's absolute accuracy, based on observations and estimations he made in the worst possible environmental conditions, while sleep-deprived and frostbitten.

They spent 16 days of "supreme strife amid heaving waters," as the boat sailed through some of the most dangerous sea conditions in the world, experiencing "mountainous" swells, rain, snow, sleet and hail. During that time, Worsley was able to get just four solid fixes on the boat's position. The rest was "a merry jest of guesswork" to determine where the wind and waves had taken them, and adjusting the steering accordingly.

The stakes were enormous—if he missed South Georgia, the next land was South Africa, 3,000 miles farther across more open ocean.

As Worsley wrote later:"Navigation is an art, but words fail to give my efforts a correct name. … Once, perhaps twice, a week the sun smiled a sudden wintry flicker, through storm-torn cloud. If ready for it, and smart, I caught it. The procedure was:I peered out from our burrow—precious sextant cuddled under my chest to prevent seas from falling on it. Sir Ernest stood by under the canvas with chronometer, pencil, and book. I shouted "Stand by," and knelt on the thwart—two men holding me up on either side. I brought the sun down to where the horizon ought to be and as the boat leaped frantically upward on the crest of a wave, snapped a good guess at the altitude and yelled "Stop." Sir Ernest took the time and I worked out the result. Then the fun started! Our fingers were so cold that he had to interpret his wobbly figures—my own so illegible that I had to recognize them by feats of memory."

On May 8, they saw floating seaweed and birds, and then spotted land. But they had arrived at South Georgia amid a hurricane, and for two days had to fight being driven by wind onto an island they had spent weeks desperately trying to reach.

Finally, they came ashore. Three of the six men, including Worsley, hiked across unmapped mountains and glaciers to reach a small settlement. Worsley joined a rescue boat back to get the other three. Shackleton later arranged a ship to collect the rest of the men from Elephant Island, all of whom had survived their own unimaginable hardships.

But the key to all of it, and indeed the recent discovery of the Endurance's wreck, was how Worsley had fought desperate conditions and still repeatedly managed to figure out where they were, where they were going and how to get there.