NavList:

Norm Goldblatt,
You asked about Joshua Slocum's tale of finding an error in his navigational tables. He described it in "Sailing Alone Around the World" (let's shorten that to SAAW) and for anyone reading this message who has NOT read SAAW, stop right now... go to Google Books... download it and read it. In the western South Pacific he waxes poetic about shooting a lunar for finding longitude. It's a wonderfully quotable epitaph for the method of lunars (I've quoted it many times myself!).

Before I go further, let me say 'welcome, aboard'. Slocum's navigation has been a popular topic over the years for the NavList community so you've found probably the best place on the Internet to ask about this topic.

You wrote:
"I am looking for verification of, for me, one of the most extraordinary claims made by Slocum in Sailing Alone. He said that off Valparaiso dead reckoning and star sighting differed by an intolerable amount and after taking many more sightings he concluded that there was an error in his trig table. Sure enough, he spotted a discontinuity, corrected it, by, I must conclude, interpolation, and after so doing, Stars and taffrail log and 'tin clock' agreed. To me, a physicist and mathematician, this is a wonderful tale."

Physicist and mathematician?? Interested in celestial navigation?? Welcome to the party -- we have t-shirts. Well, we probably should. :)

As for proof that the story is true, no one apparently felt any need to challenge him on this in life, and his logbooks and papers apparently went to the bottom with him when he was lost at sea years later. He had met Teddy Roosevelt and at one point TR's young son Archie, with a guardian, joined Slocum on a sail from Oyster Bay to Newport. Slocum showed Archie his "pages and pages" of lunar calculations which he kept in the cabin of "Spray". Archie was quite impressed, but, after all, he was twelve years old (or thereabouts). Lunars were long obsolete by that date so even other experienced navigators would have found the calculations somewhat alien. There was a brief review of SAAW in the astronomical journal "The Observatory" in 1900, and it mentioned Slocum correcting his logarithms, with a hint of amusement, but the author did not seem inclined to inquire further and heartily recommended the book. There is, however, a comment from the editor of "The Observatory" right after the review which strikes as a dubious "wink" at Slocum's claim.

Personally, I think Slocum was just explaining away some confusion of his own. Errors in the tables at this date would have been relatively unlikely. It's probably significant that Slocum was years out of practice. He did tell his son many years earlier that he had shot lunars in the 1860s. But did he after that? Had it been thirty years? Many who have read SAAW were convinced that Slocum's tale of taking a lunar implied that he was an active lunarian, and that this explains why he didn't need a chronometer on the voyage. But really Slocum was a dead reckoner, at least for longitude, during his circum-navigation. He apparently only shot this one lunar on the whole round-the-globe trip. There's a smoking gun on this: Slocum sent a letter from more than three-quarters of the way through the trip specifically saying that he had shot "just the one". His voyage from South America to the Marquesas (sighting, not landing) was the longest he had tackled in his solo sailing. He must have been near nuts from the loneliness. He may have been near panic over trusting "mere" dead reckoning for such a long passage. Out of practice and a bit batty, Slocum may well have made an error which he corrected after "wrestling with his tables" and eventually managed to soothe his ego by blaming those tables. He wouldn't be the last practical navigator to suffer from excessive pride. Another possibility is that Slocum had learned a particular method of clearing lunars decades earlier (we know he learned from an 1850s edition of Norie), and the tables he had with him that afternoon in the Pacific used a closely related, but slightly different method. His prior experience might have convinced him that the tables were wrong.

If you're looking for a real biography of Joshua Slocum, I should warn you that there are plenty of weak ones. His son Victor wrote one, which included some mistaken comments about his father's lunars, but Victor was estranged from his father and barely knew him after the age of 16. The best biography that I have read, by a wide margin, is "Alone at Sea" by Ann Spencer. She benefited greatly from the extensive research of Walter Teller in the 1950s. Teller wrote two biographies of Slocum, but he had trouble synthesizing the mountain of material that he had collected. Spencer achieved what Teller could not. There's very little about navigation in it, but that's not the real point of "Sailing Alone Around the World" anyway...

Finally, let me add that lunars are not anywhere near as difficult as legend would have it. Any navigator could learn to shoot and clear them and many used them in the first half of the nineteenth century before chronometers became common. During the latter part of the century, lunars were converted into a form of classroom torture, made as difficult as possible, probably to "separate the men from the boys" or to "spread the curve" as a modern math teacher might put it. Myself, I've shot hundreds of lunars and cleared them every which way. Accuracy is a real challenge and above all requires a properly adjusted sextant, but the math is straight-forward unless some bad choices have been made. Let me know if you want to try some yourself!

-FER

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