Ever wondered how land-based explorers fixed their positions before the era of electronic navigation aids, like GPS?

Well, the simple answer is that - like sailors on the open ocean - they made use of sextants.

I’ve been reminded of this by reading William Barr’s excellent edition of John Rae’s unfinished autobiography (John Rae: Arctic Explorer; Polynya Press, University of Alberta 2019).

Long a fan of this extraordinarily successful explorer (see 39 and 40 below), I’ve been fascinated to learn a bit more about Rae’s use of the sextant.

Not surprisingly, Rae found (clockwork) chronometers to be of limited value on his expeditions in the Arctic as they behaved very erratically and sometimes stopped altogether. So for determining longitude, Rae made use of the ‘lunar distance’ method - which involved measuring the angular distance between the moon and the sun (or certain fixed stars).

Lunar distance tables that contained predictions of these ‘distances’ were included in the Nautical Almanac. These tables enabled navigators anywhere on the planet to determine the time at Greenwich - without the help of a chronometer. By comparing this with the local time (also determined by sextant observations) it was quite easy to compute your longitude. I discuss this subject in depth in my book SEXTANT, in case you’re interested in the details.

I was fascinated to learn from one of Rae’s letters (22 September 1849 to Sir George Simpson) that the exact locations of the Hudson’s Bay Company’s outposts on the Yukon (Forts Yukon and Selkirk) were still uncertain at that time. Rae didn’t have access to a sextant and requested that one be sent to him at Fort Simpson on the Mackenzie River where he was about to become the ‘bourgeois’ - senior manager with responsibility for all the HBC’s outposts in the far north, including those on the Yukon. He proposed to make use of lunar distance observations to determine the longitudes both of Fort Yukon and Fort Selkirk.

In fact, it turned out that Fort Yukon was inside the territory of the Russian American Company! When the US government bought Alaska from the Russians in 1867, they sent an army officer, Captain Charles Raymond, to determine where Fort Yukon was. His survey showed that it was in American territory and the Fort therefore had to be moved east.

I’m not sure yet whether Rae had already found this out. But if so, the HBC must have decided to keep quiet about it!