Navigators in those days had to know their stuff and I imagine the navigator on Shackleton's expedition would have been a cut above excellent, his bio (see the wiki link in AKSAR's post) seems to confirm. I wonder how precise his sextant was capable of measuring... Point being that he used celestial to navigate close enough to get a visual on South Georgia. It doesn't really matter if they were to the left, right or on centerline; they were close enough to get a visual "through mists and squalls" -- close enough. Helluva feat.