The problem with dead reckoning in the Southern Ocean is that none of its inputs are stable. Speed varies with the sea state, which in the Drake Passage in April varies enormously and continuously. Heading varies too: a small boat in large seas does not hold a constant course; it is pushed by waves and wind in ways that accumulate into significant navigational error over time. Currents, which are difficult to measure from a small boat, add or subtract from the apparent track. And the time calculation depends on the accuracy of the chronometer, which needs to be checked against celestial observations to remain accurate.
Worsley had one chronometer. He had four opportunities to check it against the sun in sixteen days. For the rest of the passage, he was calculating — not guessing, but calculating, with the accumulated skill of a lifetime’s navigation — where the boat was, where it had been, and where it was going.
The target was an island 800 miles away that was approximately 30 miles wide at its widest point. In the latitude they were sailing — about 57 degrees south — the curvature of the earth and the length of the route meant that a navigational error of even a few degrees could put them dozens or hundreds of miles off their intended landfall. South Georgia is surrounded by smaller islands and reefs; hitting the wrong part of it in hurricane conditions would have been fatal. Worsley had to hit it correctly.
He did. The sighting on May 8 — land, after sixteen days of open ocean — confirmed not just that South Georgia was there but that Worsley had found it. He had been approximately where he calculated he was, through sixteen days of dead reckoning interrupted by four sun sights in conditions that made taking a sight a physical ordeal.