NavList:

George and Brad,

Thank you very much for your current exchanges describing your joint efforts undertaken to "re-work" an Historical Lunar with the help of the same Historical Nautical Almanac which was used by M. Bayly then.

I do not think that - except the very interested readers among us - it is necessary for each of us to fully and personnally rework all your current Lunar clearing computations with this 1773 Nautical Almanac.

However and for our Community, THIS IS AN EXCELLENT EXAMPLE OF ALL DIFFICULTIES AND TRAPS LINKED TO THE LUNAR MANUAL/TABULAR COMPUTATIONS, ESPECIALLY IF AND WHEN THEY HAD TO BE - NECESSARILY - CARRIED OUT WITH LOCAL APPARENT TIME AS A "SIGNIFICANTLY IRREGULAR" TIME VARIABLE THEN.

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As George rightly pointed out, once you had determined your Greenwich Apparent/Mean Time through painstakingly clearing your Lunar, you were far from finished with your Longitude determination trials and tribulations. You then just had in your hands only a decently updated "Time refreshed clock", and you were to use its Time Information for additionnal computations and maybe additionnal observations downstream.

In other words :

- you needed to observe your Local Apparent Time (which was not always an immediately/readily achievable observation) and such LAT " presumably was obtained from the Sun altitude " as George earlier indicated, and

- you also needed to keep/maintain your best accurate knowledge of the Greenwich Apparent Time until and for the very same instant at which you performed your Local Apparent Time observation.

As just earlier mentionned you had to keep your "best determination" of your "Lunar refreshed" Greenwich (Apparent/Mean) Time for the exact instant of your Local Apparent Time observation. So you needed a time-keeper. When Almanacs readily gave you Greenwhich Apparent Time, that was fairly simple especially if you could validly assume that during this elapsed time the EOT had not changed. When Almanacs subsequently gave you Greenwich Mean Time, then you had to perform the applicable EOT Correction with the right sign !!! (Interestingly enough, the EOT signs were the opposite between the HM Nautical Almanac, and the "Connoissance des Tems" printed in the Kingdom of France... ), and

... only at that point could you then and at last deduce your observed Longitude through direct comparison between your observed Apparent Time and your "best reckoned / best kept" Greenwhich Apparent Time for that very same instant.

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The lines hereabove are only a "refresher" for readers having only marginal interest into Lunars.

However, they are just a reminder - maybe a true discovery for some of us - about the EXTRAORDINARY DIFFICULTY to carry out all such computations at sea on a moving vessel. There were so many opportunities for blunders, that it is ABSOLUTELY AMAZING that they so consistently kept performing their Lunars and Longitude computations almost to such "perfection" given the tools they had at hand ! They were certainly close to perfection, as we should soon be able to verify when George has indicated to us the accurate geographical coordinates of the Ohama Reno Harbour.

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It brings my reflexion to another point now.

It has always stunned me that the Marcq Saint Hilaire method was discovered so late in CelNav History. For us it is so simple, so much easier than Lunars.

A possible explanation might be this one :

For decades, if not centuries before MSH, Positions computations were and HAD TO BE carried out separately for each coordinate :

- an (easy) computation for Latitude, and

- a dreadfully difficult one for Longitude.

On top of that, just add that - until the 1830's or so - they kept playing with a "non uniform" time variable, i.e. the Apparent Time which had for effect of providing them with so many additionnal opportunities for blunders ...

I think that any kind of MSH method could not have emerged until the Lunar calculations definitely belonged to History. Navigators must have been fed up then with their extraordinary complex Navigation computations until they could definitely do without Lunars thanks to reliable Chronometers, and also until thay were given the opportunity of using Greenwich Mean Time instead of Local Apparent Time. Only by that period could they and did they have enough time to "relax" and think of "inventing" new ways of tackling Celestial Navigation.

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Anyway, these Aug 1773 Lunar clearing computations currently shared from their offices to-day between Brad and George with the very same Nautical Almanac used by M. Bayly or RN Captain Cook, just keep forcing me to once again salute the EXTRAORDINARY (computing) PERFORMANCE such Men were then able to carry out at sea, day in and day out.

Antoine M. "Kermit" Cou�tte

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