NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Lunars with a wooden octant in 1825
From: Frank Reed
Date: 2007 Nov 04, 22:21 -0500
From: Frank Reed
Date: 2007 Nov 04, 22:21 -0500
Here's an article published in 1847 discussing the uses of lunars and chronometers. The principal lunar story actually refers to events in 1825. Apparently, this navigator had good success taking lunar observations with a "quadrant" which in that era would have meant an ordinary wooden octant. It's interesting that he notes how lunars were much less useful in Atlantic voyages for the simple reason that the weather is frequently bad. He also makes clear that the principal benefit of chronometers is economic. And that's worth remembering: navigation is a practical art; people adopt new tools because they give them a practical advantage... (found via Googlebooks) "By Capt. John S. Sleeper, Editor of the Mercantile Journal. Few instruments hove ever been invented, that are more ingenious or useful than the chronometer, and the improvements introduced into its manufacture within the last quarter of a century, are such as to make it an almost perfect measurer of time. The difficulties caused by the expansion and contraction of metals in different degrees of temperature, after a long series of experiments, have been almost entirely overcome�and by means of this little instrument, the longitude of a place may be determined with the greatest ease and almost perfect accuracy. The advantages of this instrument in navigation are of course immense, and begin to be generally appreciated by the mercantile community. It must be evident that the safety of a ship, and the time occupied in a passage, must in a very considerable degree depend on the knowledge which the master may have of the position of his ship from time to time. This, it is well known, cannot be determined with a sufficient degree of accuracy by dead reckoning�and before chronometers were introduced, no other means were ordinarily used at sea for this purpose than lunar observations�the process of working which in those days was exceedingly tedioue and laborious, and required much care to avoid error. The process, however, which is now used, is much more simple, and requires fewer figures than the former mode. But the great advantage which the chronometer possesses over the sextant, in determining the longitude at sea, is, that it may be used at all times when the sky is so unclouded that an altitude of the sun in the morning or the afternoon may be observed. It is not unfrequently the case that no opportunity will occur during a long voyage to Europe, of measuring the distance between the moon and the sun, or a star�while an altitude of the sun in the forenoon or afternoon, may be obtained on almost every day during the passage. Hence a chronometer on board our European traders, is not only an article of great convenience, but should be regarded as an instrument which cannot he dispensed with. Some of our West India traders also find it of great value. With a chronometer on board, a vessel with a perishable cargo can be navigated directly towards the port to which it is bound, instead of proceeding so far to the eastward that it will require several days to run down the latitude, as is too often the case. When it ie considered that a few days' difference, in a passage to the West Indies will sometimes make a difference of thousands of dollars in the sale of a cargo, the great advantage of having a chronometer on board will be at once perceived. We were once informed, by an intelligent ship-master, that he was bound to a port, St. Pierre, in one of the Windward Islands, and in the latitude of 28�, was steering due south, having already arrived to the eastward of his destined port. At this time he fell in with a lumber-loaded vessel from some port in Maine, bound to the same place, which was steering S. E. by E., the captain of which was exceedingly anxious to get far enough to windward. Our friend reached Martinico in safety, after a very short passage.�discharged his cargo, received another on board, and was in the act of leaving the harbor, when the vessel which he had previously spoken arrived! We conceive chronometers to be of much greater service in voyages to Europe and the West Indies, than in voyages to the East Indies,-although in the latter case it is well known that they are exceedingly useful. In East India voyages, during a very considerable portion of the passage, the sky is generally so unclouded that a sextant may be used, and the longitude ascertained with great certainty by means of lunar observations. When a sextant is not on board, or is out of order, a good quadrant will supply its place. In the year 1825, while on a voyage from this port to Batavia, we ascertained to our great regret that our sextant, a new one, and high-priced, was a worthless instrument. There was no chronometer on board, and we at first anticipated some difficulty in ascertaining the longitude, but we soon found that by measuring distances of objects on each side of the moon, with a quadrant, and by taking the mean of the observations, in this way the longitude could be determined, as often as was necessary, with almost perfect accuracy �this was proved in running for the Islands of Cape de Verd, the Islands of Trinidad, St. Paul, and Java Head�and so far from occupying unusual time in performing the voyage, as we apprehended might be the case, when we first discovered the worthlessness of the sextant, the whole voyage to Batavia and back to Boston, with full cargoes both ways, was performed in a space of time unprecedentedly short�being only seven months and eighteen days. The greatest objection to the use of a chronometer is, that the instrument being of delicate construction, is easily affected by injuries�and will sometimes lose its rate, and may thus deceive the navigator, and lead him into danger. This should be guarded against with the utmost care; and where a vessel is furnished with only one chronometer, it should never be implicitly relied on. Every opportunity should be seized to test its correctness�particularly by lunar observations. In European voyages, where, as we have before stated, an opportunity for taking lunar observations seldom occurs, two chronometers, or a chronometer and a well-regulated watch, will be found of incalculable value. So long as there is no essential difference in the Greenwich time indicated by these instruments, the navigator may run boldly, relying, with the aid of a good look-out, upon the correctness of his longitude�but should a variation occur, he will immediately perceive it, be upon his guard, and enabled to escape dangers which might otherwise befall him. " -FER http://www.HistoricalAtlas.com/lunars --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---