NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Mid XIX century Nav
From: George Huxtable
Date: 2005 Nov 15, 11:28 -0000
From: George Huxtable
Date: 2005 Nov 15, 11:28 -0000
Alex wrote >I am reading a book by a famous Russian XiX century > author, Goncharov, on his voyage to Japan in 1853/4. > ...The Russians asked for a > permission to land on some rock between their ship place > and the land. Apparently a small uninhabited island. > The reason they clamed for this permission was the > "Necessity to check chronometers". > Apparently they thought that this was a good reason > for landing on a rock). > > Unfortunately the author of the book was not interested > in navigation and did not want to explain to the readers > this point. WHY did the Russians think that this was a > legitimate reason? > > Why exactly did they need land to check their chronometers? > Or, more precisely, why did they think that "checking chronometers" > is a legitimate reason for a landing? > > What was the proper procedure of "checking chronometers" they had in mind? > Was this by the Lunars? > Did they mean that a firm ground > is needed for the Lunars? Or Jupiter satellites? to install a powerful > telescope? ================================ Response from George. There were things you could do on firm ground that couldn't be done from a ship, even at anchor. I will presume, for short, that Russians had then adopted Greenwich Time, without being certain that they had. In general, even the best chronometer will not keep perfect Greenwich Time. There are two important things that need to be known. Unfortunately, there's often confusion between them, because the words 'fast' and 'slow' are used for both. First, what is its error on GMT at the moment? It will be leading or lagging on GMT by an amount, which needs to be known to within a second or so for good navigation. Second, what is its rate of gaining or losing on GMT? Usually, it will not run exactly 24 hours in a (mean Greenwich) day, and the mariner was not expected to adjust it to do so; that was discouraged, even forbidden. The rate will have been previously set, with some precision, by its maker or adjuster, on land, as near to correct as possible. The mark of a good chronometer was not that it kept perfect time, but that it had a CONSTANT rate, whatever the weather, which would apply over a whole voyage. The navigator's duty was to 'rate' it, to measure how much it was diverging from GMT, whenever he could. Then he could apply that rate, over the weeks or months of a forthcoming voyage, to predict what the time error on GMT would be from one day to the next. If the vessel would be returning to St Petersburg, an immensely long voyage lay ahead. So it was necessary to establish the daily rate. That rate will have to be multiplied by the number of days into a voyage, so the best possible precision must be applied in rating a chronometer, to measure its daily rate to within a tenth of a second, or perhaps to a few hundredths. How is the mariner to do that job? Deducing the moment of noon, from Sun altitudes measured during a day, isn't precise enough to rate a chronometer over a period of a few days; it would need a long stay. The answer lies in the stars. If the navigator goes ashore, and sets up a post with a clear view of the Southern sky behind it, and some sort of peep-hole to position his eye well in front of it, he can time low-altitude stars as they disappear, instantaneously, behind that post. The interval between passages of the same star is exactly one sidereal day, which is just 3 minutes 55.91 seconds (and umpteen decimal places) short of a day by GMT. The sightline doesn't need to point due South, the post doesn't need to be precisely vertical. The chronometer doesn't need to be taken ashore, if some signalling procedure, by sight or sound (gunshot?) can be devised. The main error is in relating the observed star-time to the dial, and the ticks, of the chronometer.. It was by such methods (using a window-frame and a nearby chimney) that Harrison would test his clocks and chronometers, and how Cook would re-rate his, during a stay in Queen Charlotte Sound, New Zealand. Note that this technique finds the rate precisely, but doesn't determine the chronometer's error, for that day, on Greenwich Mean Time. That error needs to be known, but not to such high accuracy as the daily rate; to within a second or two, ideally. However, such precision did not become possible over great distances until the advent of cable, and later radio. If that sightline had been set up to be precisely South (which cam be done by methods that have already been discussed on this list) then that would allow an accurate time-sight on the Sun, or better, a star. But that would do no more than determine local time, not Greenwich Time. If the longitude of the spot was already precisely known (but likely, it wasn't), that would allow Greenwich Time to be determined. Otherwise, the navigator had to determine GMT, as accurately as he could. That could be done by lunars, averaging a long sequence of observations, which could be taken on board ship. But the best and quickest method was to time, by the chronometer, the disappearance of one of Jupiter's inner satellites into the shadow of the planet. That requires a high-magnification telescope, which had to be mounted on firm ground. It was not possible to observe on-board, even at anchor. There remained a few seconds of uncertainty, because the moment when the last glim disappeared depended somewhat on the light collection of the telescope, and on the observer's eye. It was the best method at times when Jupiter was being cooperative. However, there were whole months when Jupiter was too near the Sun to be seen, and even when it could, many of the satellite immersions occured in an observer's daylight. =========================== Alex adds- > P.S. I am well aware that the first underwater transoceanic cable layings > were made at about the same time. So the Lunars were doomed, already then, > even as a > mean > of checking chronometers:-( =========================== Not really, no. You couldn't pick up timing from a cable when at sea! Only a limited number of ports became linked to the cable network, and others had to wait for radio to come. It depended on the potential for lucrative traffic. Lunars were doomed, when chronometers became cheap enough for vessels to own a set of three, so if one went wrong it was obvious, and you could still get the time from the other two. Before the days of radio, that was an important matter. Until the days of extensive cable networks, longitudes were compared by sending a batch (sometimes as many as 20) of precise chronometers, accurately rated and timed. They went to and fro between measurement stations, often on many repeated trips, to compare with local time at each end. Fred mentioned cable-laying vessels. These needed their chronometry to be particularly accurate, because they wanted to know exactly where their cables would lie, in case (as often happened) they needed to be fished for later if they failed. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.