Lunars v Chronometers v Dead Reckoning

There’s been a few discussions here about the value of celestial navigation in the event of the loss of GNSS / GPS but not much support for the humble DR.

This is from *“Revolution in Time”* by David S. Landes:

No chart would mean a long apprenticeship on that particular voyage and a prodigious memory.

That would be true if there were no other readily available methods but navigators at that time had chip logs for speed and a compass for direction. They could use the sailings, traverse tables and like for the computations and written sailing directions as an aid.

The voyage across the N. Atlantic is short compared to some voyages and the targets are relatively large. Be hard to miss Europe when eastbound or North America westbound.

Columbus made four trips using (mostly) dead reckoning, the first with no pervious experience.

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Good book on early transatlantic trade is “The Tobacco Coast” rich in Chesapeake nautical detail from 17th century. One item I remember is early voyages from England to Virginia Capes masters used soundings to determine how close they were to coast line.

Nothing unusual about that before GPS & inexpensive Radar soundings off the flasher one way most small boats navigated coastal & inshore.

To find the 15 mile wide Chesapeake entrance early navigators armed the leads hollow base with tallow. Composition of the bottom told them they were near the entrance. Bay discharges tons of sediment, sandy low islands north and south don’t. Dark sediment told them they were in close proximity to the Capes.


Dead reckoning is reliable on coastal passages as long as there’s passable visibility. Not quite as reliable on open ocean passages but still gets you within radar coverage of the coast you’re aiming for.

Without radar we used DF and soundings. During the winter months there was not much in the way of sights so it was mostly by dead reckoning.

I remember using AM radio stations as DF navaids in my early bush flying days. Old enough to remember that.


Mid 70s I was a coxswain on CG Utility boats lower Chesapeake. Most often 30 footers compass tachometer sounder and a RDF you could not trust. Assistance towing always back to same 2 inlets. Carried a bearing & distance shirt pocket book with notes, courses from various buoys . Poor visibility run south, cross the main ship channels depth drops off. Find the edge then run east or west until passing a bouy. Lot of set depending on the tide. Read the number look at the book and run the course to suit.

Today GPS goes out majority of amateurs can’t find their way home. Back then frequent assistance call was lost boater by one of the bridges islands. Run out, find him, few circles with the blue light and siren pick up several other’s who followed me back home.



There was a pop radio station in California that was within the frequency range of our RDF set that as second mate I used to use to check what time zone was flavour of the month back in the day. I seem to remember that the RDF set had been deliberately designed to prevent mates listening to music.The DF log book was always a bone of contention when we had Board of Trade inspections, never saw it looked at by class.

The music and the DJ’s banter and mandatory periodic station ID was confirmation we had the right station.

IIRC you needed ADF to hear the AM station broadcast but the RDF antenna only pointed to it.

The Napoleonic Wars were 1803-1815. Crossing the Atlantic at that time without a sextant or chronometer was certainly doable but I don’t know if it was actually commercially practical or not.

In Sailing Alone Joshua Slocum says the error using DRs was 5 miles a day or more. The internet says New York to Liverpool was about a 25-30 day passage which would be a possible error at landfall of 150 miles or more.

Don’t know about actual practice, maybe an experienced navigator could improve on that. Also don’t know what the cost of the purchase and maintenance of a chronometer was at the time compared to the cost of the added distance and risk of the voyage.

Seems like a sextant for noon sights would be relatively cheap.

John Harrison revolutionized navigation when he invented the world’s first chronometer in the 1730’s but it wasn’t developed into a practical instrument for use aboard ships until later in the century. A book titled Longitude written by Dava Sobel details his struggle to be taken seriously.

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The “Gould” mentioned in the OP was Rupert T. Gould author of “The Marine Chronometer: Its History and Development” first published in 1923. The chronometer had to compete with lunars and at least on shorter voyages the DR.

Gould literally cleaned Harrison’s clock:

He gained permission in 1920 to restore the marine chronometers of John Harrison, and this work was completed in 1933.

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From Global Ocean of Knowledge, 1660-1860: Globalization and Maritime Knowledge in the Atlantic World

The world’s Navies adopted Chronometers relatively quickly.


The merchant side not so much.


And the hold-outs, Joshua Slocum with his famous “tin clock” in 1895.


$15 (over $500 today) was probably more than he spent fixing the hulk given to him for free. An extravagance for a Bluenose.

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Slocum had $1.50 when he departed and he was an expert at lunars. Spending $15 for a chronometer didn’t make sense, at the margin.

I found from the result of three observations, after long wrestling with lunar tables, that her longitude agreed within five miles of that by dead-reckoning. This was wonderful; both, however, might be in error, but somehow I felt confident that both were nearly true, and that in a few hours more I should see land; and so it happened, for then I made out the island of Nukahiva, the southernmost of the Marquesas group, clear-cut and lofty. The verified longitude when abreast was somewhere between the two reckonings; this was extraordinary. All navigators will tell you that from one day to another a ship may lose or gain more than five miles in her sailing-account, and again, in the matter of lunars, even expert lunarians are considered as doing clever work when they average within eight miles of the truth…

The result of these observations naturally tickled my vanity, for I knew it was something to stand on a great ship’s deck and with two assistants take lunar observations approximately near the truth. As one of the poorest of American sailors, I was proud of the little achievement alone on the sloop, even by chance though it may have been…

The work of the lunarian, though seldom practised in these days of chronometers, is beautifully edifying, and there is nothing in the realm of navigation that lifts one’s heart up more in adoration.

Doing lunar distances back then is somewhat analogous to what celestial navigation is today.

Even with GPS receivers on board I’ve always found comfort in being able to rely on a sextant if they went south for whatever reason.

Sure, it’s a cost / benefit thing. Sextants are relatively cheap.

Well said. There’s a certain amount of blind faith needed in the delivery business. A sextant is cheap insurance.