Slocum's chronometer

[chabrenas]

Thought it might be better to start a separate thread than discuss this on the very interesting existing thread on what to use as a clock in our day and age:

Joshua Slocum claimed he used an alarm clock with only an hour hand, which he boiled in the kettle whenever it got stuck. Nice joke from an experienced master mariner, but how did he really navigate?

 

[dulcibella]

I could have this wrong, but my impression was that he only used the alarm clock to get his approximate sight time (eg for meridian sun). Before chronometers came into use, longitude was determined (if at all) by a lunar sight technique that required much skill and was somewhat error-prone - I thought that Slocum did it this way. Perhaps someone who is more than an occasional astro-navigator can give a better explanation.

 

[Mark-1]

Thought it might be better to start a separate thread than discuss this on the very interesting existing thread on what to use as a clock in our day and age:

Joshua Slocum claimed he used an alarm clock with only an hour hand, which he boiled in the kettle whenever it got stuck. Nice joke from an experienced master mariner, but how did he really navigate?

I read his book in the Summer. I think he only calculated his longitude once or twice on the whole trip.

I'm sure he actually says at one point that people of his time (including him) were too dependent on an accurate position and that a decent position is no substitute for a decent lookout.

His book is online at www.bluemoment.com so someone can check my memory is correct. (It likely isn't!)

 

[Salty John]

Slocum used the old method of Lunar Distances to calculate his longitude. This doesn't require a clock.

 

[snowleopard]

Slocum used the old method of Lunar Distances to calculate his longitude. This doesn't require a clock.

A common misconception it seems. Yes he did fix his position by Lunar Distances a few times but it was not his regular method of navigation. He mostly relied on latitude and DR.

Remember that up till 20 years ago, position uncertainty was a way of life at sea. Approaching a coast with only a DR position was routine.

 

[Salty John]

Misconception? I have no idea what Slocum's regular means of navigation was. I was simply answering the earlier question regarding his ability to determine a position without a chronometer - he used lunar distances. Seemples.

 

[prv]

I'm reading the book intermittently at the moment. He said he owned a chronometer (he was a career seafarer before Spray) but at the time of the voyage it was out of calibration and he begrudged the $15 (I think) to get it checked and adjusted. So he went to sea with a cheap "tin clock", I think donated or sold by someone in his port of departure from the US.

Brings into perspective anybody wasting sailing time waiting for "vital" equipment to be fixed

Pete

 

[earlybird]

Slocum used the old method of Lunar Distances to calculate his longitude. This doesn't require a clock.

Interesting, that a single-hander in a small boat can apparently use this method satisfactorily.
Why then did the Admiralty seem to place such importance on the development of the chronometer? This is always presented as a tremendous breakthrough, rather than just an improvement.

 

[Lady Campanula]

Interesting, that a single-hander in a small boat can apparently use this method satisfactorily.
Why then did the Admiralty seem to place such importance on the development of the chronometer? This is always presented as a tremendous breakthrough, rather than just an improvement.

It may be worthwhile reading the Wikipedia entry, especially the section on 'Errors'.

It will be seen that an expert navigator with a fine sextant could fairly reliably produce a LOP reflecting the vessel's Longitude, in good conditions, to about 15 miles. This was still combined with (an)other LOP by 'sun-run-sun' to produce a fix, which was then used to correct the continuous DR plot.

Most master mariners were not that expert in manipulating spherical trigonometry with the confidence needed, nor were the sextants most had access to and used in practice necessarily the best available. Actual weather and observational conditions were frequently far from optimum.

It will readily be seen that, in practice, the Terminal Error of a plot dependant on Longitude by Lunar Distance could frequently be considerably in excess of 15 miles. As the objective was making a safe landfall in thick weather - 15-20 miles error on mid-ocean usually matters little - and the avoidance of shoals and isolated rocks such as the Wolf Rock, The Barrels and Hats and the Isles of Scilly's Western Rocks, one can see that a more accurate and more reliable method was highly desirable.

Hence the Board of Longitude's prize for the first reliable and accurate chronometer and Dr Maskelyne's duplicity...

at a meeting of the Board of Longitude in early 1765, where it was disclosed that Harrison's chronometer had produced Bridgetown's longitude with an error of less than ten miles after a sea voyage of more than 5,000 miles. Maskelyne's Lunars method on the other hand showed an error of 30 miles.

While the controversy over which was best raged for some time - like an 'anchors' or 'GW' thread on here - support by navigators swung inexorably behind Harrison's Chronometer and the short-lived Lunar method faded into history.

The last known practical use of a Lunar Distance sight reduction was on the ill-fated Apollo 13 Lunar Module, when a modified form of 'Lunar' was used in the crippled spacecraft to help determine when the 're-entry burn' should be initiated.

 

[cliffordpope]

Interesting, that a single-hander in a small boat can apparently use this method satisfactorily.
Why then did the Admiralty seem to place such importance on the development of the chronometer? This is always presented as a tremendous breakthrough, rather than just an improvement.

Probably because wrecking one lone eccentric in an old boat mattered less than piling their expensive new battleship onto the rocks.

 

[JayBee]

Google "Sir Cloudesley Shovell" for interesting stuff on the search for the longitude.