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    Re: Advice concerning sextants
    From: Gary LaPook
    Date: 2011 Jan 5, 20:43 -0800
    Read the third paragraph from the end.

    gl

    --- On Wed, 1/5/11, Gary LaPook <glapook---.net> wrote:

    From: Gary LaPook <glapook---.net>
    Subject: [NavList] Re: Advice concerning sextants
    To: NavList@fer3.com
    Date: Wednesday, January 5, 2011, 8:35 PM

    I think he was recommending that you work the sight as a normal LOP by using the tables, HO 249 or HO 229 or others. You should get the same latitude since the resulting LOP will run east and west for a noon shot.
    gl

    --- On Wed, 1/5/11, Patrick Goold <goold{at}vwc.edu> wrote:

    From: Patrick Goold <goold{at}vwc.edu>
    Subject: [NavList] Re: Advice concerning sextants
    To: NavList@fer3.com
    Date: Wednesday, January 5, 2011, 7:48 PM

    I am getting good LAN sights now.  Today I was within a tenth of a minute of the GPS latitude and two-tenths two days before that.  I think it is time to move on.  Greg, you have indicated a path forward, but I do not understand what you mean.  Can you elucidate?

    Patrick

    On Thu, Dec 30, 2010 at 3:39 PM, Greg Rudzinski <gregrudzinski---.com> wrote:

    Patrick,

    As a check try reducing the LAN as a regular LOP observation to hopefully get an azimuth of 180� and an intercept from GPS to match the LAN Latitude.

    Greg Rudzinski


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    --
    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

    Charles Olson: "Love the World -- and stay inside it."

    Ad hoc celestial teacher on Royal Clipper

    Dec 04, 2009 12:00 am

    Recently, one of our readers, Gary LaPook, sailed transatlantic on the square-rigged cruise ship Royal Clipper. Here is his report:
    We recently crossed the Atlantic on the Royal Clipper and I recommend it to anyone with an interest in the great windjammers. It is not cheap but a good value considering the number of days you get for your money. We boarded in Lisbon on October 22nd and stopped in Casablanca and Safi, Morocco and at Tenerife, Canary Islands. Then we were ten days at sea and arrived at Barbados on November 7th. She is 437 feet long, has 26 square sails on five masts and 16 fore and aft sails. There are five sails on the fore and mizzen, six sails on the main and middle masts, and four sail on the jigger mast. Of the 26 square sails, 24 roller furl inside the yards but two, the main course and the crossjack, use the traditional clew garnets, bunt lines, leach lines, and gaskets for handling the sails and crewman had to go aloft to work these sails. I asked the captain and he said they kept these two so that they could demonstrate the old ways to passengers. Bracing the yards is done with hydrolic brace winches mounted at the base of each mast, two per mast, one pulls in on the braces while the other pays out. Each winch has two braces led to it but onto a drum that has two different diameters. Because of the way the braces are rove, each brace moves up to three of the yards at the same time so that one brace winch, with the two braces led to it, can brace all six yards on one mast. This eliminates the large manpower needed in the olden days when each yard had its own set of two braces.

    She is 437 feet long but takes only 220 passengers and the ship and its appointments are more like a yacht than a cruise ship. You can climb the ratlines to what they call the crows nest but would traditionally be called the top at the level of the main course yard and you get a great view from there. You can also go out on the bowsprit or lay in the net under it and watch and hear the sea rushing by under you. We met many people and most of them had made this voyage many times (some, many, many times) and we are planning to do it again, hopefully next year.

    We were under only sail about half the time and we got up to eleven knots solely with sail power. But when the wind was light, usually at night, they would proceed under power to keep to their schedule. I looked over the stern and could see when the prop was driving the ship and the ship had a different feel when under sail alone, a wonderfully different feel.

    The ship published a schedule for each day. On the second day they had sextant use and the noon sight on the schedule. I went to the bridge at the scheduled time and found passengers looking through about ten Davis sextants looking at the sun. Not surprisingly they were getting many different readings and I looked through several of the sextants and they all had significant index errors, one of three degrees! I was disappointed that no one was explaining what the readings were used for. That afternoon the schedule said advanced navigation so I went to the bridge for that. One of the ship's officers was explaining very basic stuff to about a dozen passengers and I looked around and found H.O. 249 on the bookshelf. Someone asked about variation and the second officer pulled out a blue book and said that the changes in variation were listed in that book. From where I was standing the blue book looked like the nautical Almanac and I had never noticed variation information in that book. I innocently asked the officer if, when doing celestial, he used H.O. 249 or if he used a calculator or programed computer. He asked me what H.O. 249 was. I figured that maybe he knew it by some other name since he came from the Ukraine so I got volume 2 off the bookshelf and showed it to him. He flipped through several of the pages and had that deer in the headlights look and it appeared he had never seen the tables before! Even if he was not familiar with this particular set of tables I would expect any navigator to recognize a sight reduction table even if it was not the exact same set of tables that he was used to using. He then said no, everything is in the blue book. So much for the level of celnav being practiced today.

    I had brought my Tamaya along and planned to take noon sights to gather data for our discussions and to take as many sights as I could to gather data on the accuracy obtainable with celnav (I will write about these things later) so the next day I was on the bridge at noon and took a noon sight. Another of the ship's officers saw me and asked me if I would teach him celnav and I agreed and ended up with about eight other students the next day to take the noon sun. I let them each take a sight as the sun was approaching noon and after it started descending we worked out the latitude the traditional way and it agreed within one NM with the GPS, not bad for first time sextant users. I told them that this was a special case with very simple arithmetic for the solution.

    That evening I hit upon a way to introduce the general solution by the St. Hilaire method. I made up a plotting sheet and drew a line showing the latitude line we had determined at 24 degrees 49 minutes North. I then explained that another way we could have done it was to work the standard noon formula from the other end and compute what we would have measured if we had been at 25 degrees North. Then comparing the observed altitude and the computed altitude and knowing the azimuth of the sun was straight south we plotted the LOP and showed that we got the same latitude as before. I then got H.O. 249 and showed that we got the same result using it with an LHA of zero. I then showed them that H.O. 249 would do the computation for the general case. I have never seen it taught this way and I had never thought of it before and I recommend this method to you when you teach celnav.

    The next day we had the group shoot the afternoon sun and then we stepped through the entire solution. I had to make up forms for them to use and they were not perfect so there were many math errors due to the unfamiliarity of dealing with degrees and minutes and keeping them in the correct columns, good forms would have made this easier. We then plotted each of their LOPs on a plotting sheet and compared them with the GPS values.

    Near the end of the voyage I was talking to the captain and he told me that he does the calculation on a normal calculator and he challenged me to a competition. Later that day I showed up on the bridge with my calculator in my pocket and said I was ready for the competition. He made up a set of values but when he got out his calculator he said it was kaput the battery was dead. I worked the problem, so I guess I won. This gave me a great segue to the flat Bygrave. I got it out of my cabin and gave it to the captain as a gift, with some forms, and told him the batteries will not go bad on this one.

       
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