NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Use of Sun Sights for Local time, and Lunars for Longitude
From: Arthur Pearson
Date: 2002 Oct 6, 21:44 -0400
From: Arthur Pearson
Date: 2002 Oct 6, 21:44 -0400
List Members, I posted the email below to the list but it never stuck. I was primarily addressed to George's comments on the recent thread regarding "Variation of compass", so I forwarded it to him directly. As other members of the list may be interested, I am posting it again. My original email is immediately below, and George's very thorough response follows it. Arthur ORIGINAL NOTE: George, et. al., This question pertains to the calculation of Azimuth and Local Time from a sun sight, and the use of this technique in conjunction with lunars. Without developing the formulas, I can see that if we have Latitude, Altitude and Declination, we have all sides of the triangle. We can then compute Azimuth and LHA, and by converting LHA arc to time, we can get the difference between observation time and the time of Local Apparent Noon and thus Local Time (please correct any inaccuracy or imprecision). Having made these calculations, I can now take a lunar distance and derive GMT (so much more easily said than done!). Was the determination of longitude then simply a matter of taking the difference between Local Time and GMT and converting time to arc to get Longitude? I had presumed the purpose of getting GMT from a lunar was to correct your chronometer such that star sights and running sun fixes could be accurately reduced to get a fix that revealed both latitude and longitude. Getting latitude via LAN sight, then local time by afternoon sun, then GMT by lunar, and then calculating longitude seems a reasonably efficient alternative. I am curious what Lecky's indicates was general practice during the time that lunars were in common use. One final question: I am not sure why Local Time would be used to set the ship's clock as Local Time changes with Longitude. When I have crewed on ships, the Chronometer was set to GMT and the error and rate documented at least daily, but this was in the age of radio time checks. Thanks for any light you can shed. Regards, Arthur GEORGE'S RESPONSE: Dear Arthur, You ask some sensible and relevant questions, so it would be worthwhile to post it to the list if you possibly can, so anyone else can have a crack at replying, not just me. However, I am happy to have a go. I will be thinking of the period from say 1750 to about 1870, when lunars were an important source of longitude information for mariners, though being displaced by the chronometer When you look at early navigational techniques, you have to be aware of the mindset of those navigators of the past, the way they thought, and the tools and information they had available. >One final question: I am not sure why Local Time would be used to set the >ship's clock as Local Time changes with Longitude. When I have crewed on >ships, the Chronometer was set to GMT and the error and rate documented at >least daily, but this was in the age of radio time checks. Let's start with your last question, about the ship's clock. That's not the same thing as the chronometer. As you say, on a modern vessel you would always have a chronometer set to Greenwich. But even on a modern vessel, wouldn't you always find another clock on the bridge indicating at least ZONE time, to regulate the ship's business? Zone time, however, is a rather modern concept. In the 18th century, THE TIME meant local time-by-the Sun. On land, the village church clock would be readjusted according to the sundial in the churchyard, or by an amateur astronomer with a couple of posts in the ground to establish a South transit. We know (and even the Greeks knew) that time measured that way was non-uniform: no accurate clock could be made to run at that variable rate, and the laws of physics could not be applied if time was measured that way. Each village kept its own time. Any concepts of time-at-other-places such as Greenwich, or of mean time, were quite alien notions, familiar only to the astronomers with their accurate clocks. On all ships, including those making long passages in longitude, it was necessary to keep a clock registering local time-by-the-Sun, if only to time the routine calling-of-the-watch. For example the dog-watches were always in the afternoon, and it would never do to have the dog-watches slide into the morning as the longitude altered when an ocean was crossed. At sea, it wasn't possible to time the Sun's crossing of the Southerly meridian, but it was straightforward to establish the local Sun-based time from morning and/or evening Sun altitudes, to allow the ship's clock to be reset. Because the ship's clock was never very precise anyway, tweaking the clock to adjust for longitude changes was not a very different matter from tweaking it to compensate for its own unpredictable gains and losses. It was usually done at a morning Sun sight, if the Sun obliged by appearing: otherwise, some guesswork was used. Of all our different methods of measuring time, we name that measure "Ship's Apparent Time", but to a navigator prior to the days of the chronometer, he would simply think of it as "The Time". That, and the latitude, were the two quantities he would always know well, provided his skies were clear, and without needing recourse to lunars or a chronometer. >This question pertains to the calculation of Azimuth and Local Time from a >sun sight, and the use of this technique in conjunction with lunars. Without >developing the formulas, I can see that if we have Latitude, Altitude and >Declination, we have all sides of the triangle. We can then compute Azimuth >and LHA, and by converting LHA arc to time, we can get the difference >between observation time and the time of Local Apparent Noon and thus Local >Time (please correct any inaccuracy or imprecision). Well, nowadays you might calculate the azimuth, but not in those days. Otherwise, I agree. And as you say, this would be Local Apparent Time (and not Mean Time, which has the Sun's timing inconsistencies averaged out). >Having made these calculations, I can now take a lunar distance and derive >GMT (so much more easily said than done!) Was the determination of >longitude then simply a matter of taking the difference between Local Time >and GMT and converting time to arc to get Longitude? Yes, but there's a complication. It depends somewhat on the year. Let me explain. The longitude is the difference between the Local Apparent Time and the Greenwich Apparent Time. Or, which is just the same thing, the difference between the Local Mean Time and the Greenwich Mean Time. The important thing being that the two measurements have to be made on the same time-scale. Prior to 1834, all times in the Nautical Almanac were in terms of Greenwich Apparent Time (which was, in the old sense of the word, the "argument" of the almanac). So prior to 1834, interpolating in the table of lunar distances would provide the Greenwich Apparent Time of the observation, to compare with the Local Apparent Time from the Sun, so that's easy. From 1834 the increasing importance of the chronometer (which by its nature must measure Mean Time) led to the time-argument of the almanac changing to Mean Time throughout. The difference between these two time scales can be up to about 15 minutes either way, and is tabulated in the almanac so it's easy to convert one to the other (though it's often easy to get the sign wrong!) >I had presumed the >purpose of getting GMT from a lunar was to correct your chronometer such >that star sights and running sun fixes could be accurately reduced to get a >fix that revealed both latitude and longitude. No. That is the way it would be done after the days of Sumner and St Hilaire in the mid 1800s, and how we would do it today. Before then, however, what appears to us as a simple concept of taking an altitude of any body, whatever azimuth it happeed to be at, and finding a position-line at right-angles to that azimuth, just hadn't occurred to navigators. An extraordinary lapse, it seems to me, that this concept didn't appear until so late, when all the mathematical tools existed. Instead, navigators thought of latitude and longitude as quite separate quantities: longitude being determined in a separate observation after the latitude had been obtained from a noon Sun. >Getting latitude via LAN >sight, then local time by afternoon sun, then GMT by lunar, and then >calculating longitude seems a reasonably efficient alternative. Here you summarise the position well. The GMT could also be taken from a chronometer (adjusting for Equation of Time where necessary) and perhaps the chronometer may have been checked against a lunar observation if the navigator was a belt-and-braces man. > I am >curious what Lecky indicates was general practice during the time that >lunars were in common use. Lecky came right at the end of lunars and dismisses them with some contempt. Definitely a chronometer man. >Thanks for any light you can shed. Hope it helps. If anything is unclear, or seems wrong, do come back. I find that trying to explain such matters to someone else helps to clarify one's own thoughts. If you find a way to post up your question to the list, you are welcome to include this response (or not: whichever you prefer). Yours, George. ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------