# NavList:

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

**Re: Latitude by Noon Sun for Beginners**

**From:**Gary LaPook

**Date:**2009 Apr 30, 00:20 +0200

JPK wrote: "As an aside, you asked why I thought there was a 12-minute time span between the time of the shot and LAN for the DR longitude. I converted his DR longitude of 68d 25' to time, and got 4h + 32m + 100s, or 16:33:40. I figured that meant that the sun would be over his meridian at that time. The shot was made at 16:46:20, which is 12m and 40s later. Did I go wrong on that reasoning? Did the navigator make his shot when he did, rather than when I would have advised him to had he asked me, simply because he recognized that his DR was not likely to be close enough to make the difference significant? " That is one way to do it but then you have to apply the equation of time which is listed at the bottom of each Nautical Almanac page. As Jean-Philippe Planas points out, the sun was 3� 03.1' slow meaning that it didn't cross the Greenwich meridian until 12 minutes and 12 seconds after noon Z. The equation of time shows the minutes and seconds that the sun is fast or slow compared to mean time and the almanac also lists the time of meridian passage, just another way of expressing the equation of time. The sun can be up to 16 minutes off form mean time during the year. A different way to accomplish the same thing is to calculate the time when the GHA of the sun equals the longitude. Take out the GHA for the hour when it is east of the DR and subtract the GHA from the longitude to find the difference. Then flip through the Increments and corrections section of the almanac to find our how many minutes and seconds it takes for the sun to move that additional distance. To get a start on the right page of the increments section remember that the sun moves one degree every four minutes and one minute of GHA for every four seconds of time. That said, I think that you still don't get it. You are looking through the wrong end of the telescope. For a LAN sight you do not use your DR longitude to try to predict the exact time of LAN in advance, you just get an approximate idea so you can get your sextant into operation prior to LAN. The navigator does not take the shot when advised that he should based on DR, he starts taking sights sometime prior to the approximate time of LAN and follows the sun as it increases in altitude. Then when he notices it starting down he says mark and you note the time at that point. Note, the time of LAN is determined by seeing the sun start to descend not the other way around, as you suggested, by a calculation of the time the sun should be at the DR. You then use the observed time of LAN to enter the almanac to take out the sun's declination for the calculation of latitude. To some level of accuracy you can also use the time of observed LAN to determine your longitude and the level of accuracy that can be achieved this way has been the subject of the recent threads. If you can be sure that you did not miss LAN by more than a minute then the longitude should be accurate within plus or minus 15'. If you can be certain that your possible error in noting LAN is less than one minute then your level of accuracy will be better. Even with the one minute level of uncertainty in the time of LAN the 30' range of longitudes may be completely adequate for emergency or backup navigation. In fact this level of accuracy is twice the level required to have won the top prize of 20,000 pounds from the Board of Longitude in the 18th century. Prior to the development of the chronometer navigators routinely sailed around the world with little idea of their longitude relying on latitude sailing to find their destination. The LAN sight is a special case in which the navigational triangle collapses to a line making the computation of latitude one of simple arithmetic. In the olden days, doing trig using trig tables was a major chore so navigators liked special cases like this one. Similar is the use of Polaris for latitude, just simple arithmetic. Ex- meridian was a more complicated procedure in which you made a correction of a non-LAN sight using some assumptions (so some sources of error) so that it could be treated as a LAN sight and so would still be easier that the full course press using trig. Now a days, with calculators and inspection tables and better educated navigators, I don't see any reason to continue these special cases, just treat them all as normal sights, calculate Hc and Az and plot the LOP as I pointed out in my previous post. gl JKP{at}obec.com wrote: > Gary, > Thanks very much for your in-depth explanation. I find that it was the declination giving me trouble, and I finally realized I was subtractng 30' for the "d" correction, not 0.5' as I should have. (Half a degeree instead of half a minute!) So for the time of the sight I (eventually) got a declination of 18d 59.8'S and the correct latitude of 55d 37.8'S. Whew! > > As an aside, you asked why I thought there was a 12-minute time span between the time of the shot and LAN for the DR longitude. I converted his DR longitude of 68d 25' to time, and got 4h + 32m + 100s, or 16:33:40. I figured that meant that the sun would be over his meridian at that time. The shot was made at 16:46:20, which is 12m and 40s later. Did I go wrong on that reasoning? Did the navigator make his shot when he did, rather than when I would have advised him to had he asked me, simply because he recognized that his DR was not likely to be close enough to make the difference significant? > > Or, as another reader suggested, was this perhaps meant to be an "ex-meridian" sight? In that case, as I eventually found in Jeff Toghill's "Celestial Navigation," the observed altitude should be corrected for the time difference by factors found in Bowditch. (The tables referred to were 29 and 30, though in the 2002 Bowditch they are now 24 and 25.) I plan to rework the problem from that perspective and see what I get. > > Thanks again for your patience and thoroughness. This list is like having dozens of expert tutors on call! > > John > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---