# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Sun Moon Lunars to 155 degrees**

**From:**George Huxtable

**Date:**2010 Apr 2, 16:36 +0100

Brad wrote- "It is always a possibility that multiple octants were used, of course. It was a scientific expedition and we do know (based upon earlier comments) that there were two octants on board." from George- Not quite so. We know of two SEXTANTS aboard Adventure, supplied by the Board of Longitude. These were immense, heavy, 15-inch-radius jobs, one by Dollond and one by Ramsden. These were similar to the two provided for Resolution. Additionally, we know that Resolution's officers had also, between them, at least three more (possibly four more) brass sextants of their own. If Adventure's officers had been able to muster a similar number between them, there would have been more than enough for measuring the lunar distance and the altitudes simultaneously, given three observers; which no doubt were available on such a well-manned vessel. I know nothing about any wooden octants aboard, but think it likely there would have been several, and these would probably be regarded as good enough for taking the altitudes. Bayly, in a letter to Banks about the third voyage, quoted in David vol 3, writes about the poor accuracy he found in the 15-inch Ramsden aboard Discovery, (which may well have been the same instrument used on Adventure in the second voyage) finding that it was producing longitude errors of sometimes more than a degree; "in consequence of which I made no other use of it during the remaining part of the voyage but to observe altitudes at the time of observing distances, as in that case a small error on altitude would seldom cause any effect in computing of the correction for parallax and refraction" Which appears to confirms that simultaneous measurement of altitudes with lunar distance, requiring three observers with three instruments, was the way the job was done. However, if necessary, the astronomers were capable of obtaining the altitudes of the bodies by calculation rather than observation, as Maskelyne had explained how to do so in his British Mariner's Guide of 1973. Now for Brad's comments about the various symbols that were placed by the lunar distance: no doubt this will become clear once someone can get access to the relevant pages of the Second-voyage report. In the Third voyage, which also visited Ulietea and made observations over several days, there was a complex program of different observers (Cook and King)making simultaneous measurements of lunar distance with various sextants, which they then exchanged and remeasured (see page 43, third voyage). Four Ramsden sextants, a Dollond, and a Bird were compared in this way, so many different combinations of sextant and observer occurred, and provide some remarkable results, which are well worth a bit of scrutiny. But the point I wish to make is that the various instruments were marked by one set of code-letters in that table, and the observers by another set. I suspect that the symbols used against the lunar distance in the second voyage have a similar intention, even if we can't yet decode it. George. contact George Huxtable, at george{at}hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ======================= Brad wrote- It is also a possibility that we are being presented with averaged results, over individual or combined sextants. Bowditch recommends that "Whilst one person is observing the distance of the objects, two others ought to be observing the altitudes. The chronometer should be placed near one of the observers, or put into the hands of a fourth person appointed to note the time; the observer who takes the angular distance giving previous notice to the others to be ready with their altitudes by the time he has finished his observation; which being done, the time, altitudes and distance should be carefully noted, and other sets of observations taken, which must be done within the space of 15 minutes, and the mean of all these observations must be taken and worked as a single one". I think this advice is consistent with what we are seeing in the log, as tne log shows the "No. of Obs." in the 8th column. We are seeing the result of multiple observations, averaged, to show the one distance recorded. If Cook & Co used multiple observers or not is something hard to tell. Bowditch also offers this advice: "Method of taking a lunar observation by one observer. In case of not having a sufficient number of instruments or observers to take the altitudes, it has been customary to calculate them...These calculations are long...the following method of obtaining these altitudes is far more simple and sufficiently accurate. This method depends on the supposition that the altitudes increase or decrease uniformly. Before you measure the distance of the bodies, take their altitudes, and note the times by a chronometer; then measure the distance and note the time, (or you may measure a number of distances, and note the corresponding times, and take the mean of all the times and distances for the time and distance respectively;) after you have measured the distances, again measure the altitudes and note the times; then from the observed altitudes of either of the two observed altitudes of either of the objects, the sought altitude of that object may be found in the following manner:- Add together the proportional logarithm (Table XXII) of the variation of altitude of the object between the two times of observing the altitudes, and the prop. log. of the time elapsed between taking the first altitude and measuring the distance; from the sum subtract the prop. log. of the time elapsed between observing the two altitudes of that object; the remainder will be the prop. log. of the correction, to be applied to the first altitude, additive or subtractive, according as the altitude was increasing or decreasing ; to the altitude, thus corrected, apply the correction for dip of the horizon and semi-diameter, as usual" Whew! In other words, estimate the alitude of the objects at the time that the distance was measured by using proportional logarithms! If I recall properly, and there were only two octants on board, then it would not have been possible to simultaneously measure the altitudes of two objects AND the distance between them. Further, for the example being worked for the distance in Ohamaneno Harbor, the sun's altitude could not be measured, the horizon was masked by the island. So it is possible we are being presented with a mix of cases, sometimes with the altitudes being measured, sometimes with the altitudes being calculated. ---------------------------------- From all of this, here is my first tentative assignment of the mark found by the distance, in column 3. There are four mark types, "Cross", "Doubled Cross", "Dots" and "Blank". Could these correspond to the types of altitude used in the distance calcuation. That is: (1)Both altitudes measured, (2)Sun altitude measured Moon altitude calc'd, (3) Moon altitude measured Sun altitude calc'd, (4) both altitudes Calc'd. Any takers???? :-)