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    Re: Longitude by Time Sight...good enough at sea?
    From: Greg Rudzinski
    Date: 2012 Nov 30, 11:44 -0800

    Doug,

    Try HANSEN'S IMPROVED EX-MERIDIAN TABLES. These tables are hard to find though. The reprinted 1964 edition uses LHA. The first edition 1919 uses time from MP. The advantage is speed of calculation and the extended period from MP for accurate latitude determination. This gives the navigator about a 2 hour window for determining latitude vs. a 45 minute window using factor (a) and formula (a)(t^2)/60 or .267(a)(LHA^2). Also try using the formula with a slide rule and (a) by table as a unique non electronic method.

    Greg Rudzinski

    [NavList] Re: Longitude by Time Sight...good enough at sea?
    From: Doug MacPherson
    Date: 30 Nov 2012 09:04
    Hi Michael:

    Thanks for your response. I am a mac user but your programs sound like nice work.

    I have been playing with the ex meridian and azimuth as well. Like you, I am always looking for ways to make this simple...fewer calculations, fewer books to buy etc., no electronics. I use copy of Norie's Nautical Tables for this work.It is amazing what the sailors of old could do with a copy of Norie's? I'm still digesting all the tables. For instance, I just worked through the use for the "Change of Hour Angle with Altitude" table and how it can be used with the Ex Meridian work.

    In order to try and put all my "noon" work together I created the attached work sheet that allows you to compute latitude by local apparent noon, and a LOP with the ex meridian tables and azimuth.

    Is there anything like this out there? See any improvements I could make in mine?

    Thanks!

    Doug

    On Thu, Nov 29, 2012 at 11:52 AM, Michael Bradley <mcbradley_uk---com> wrote:
    Doug

    Welcome to the 'Do It The Easy Way' section of NavList.
    In the British Merchant Marine a common development of the Time Sight was to plot an LOP using the 'Longitude by Chronometer' from the Time Sight, the chosen Latitude, and a separately calculated/looked up Azimuth. So you can use the Time Sight in LOP navigation if you wish ( see last paragraph).

    Frank is absolutely correct about the past practice of avoiding any marks on the chart at all, this particularly applied in the days when the cost of a chart was considered a capital matter - an interesting contrast with today's times when the culture of entitlement (i.e. the expectation of free charts ) is in full operation.

    One of the problems of avoiding marks on the chart is that of calculating and expressing usefully the DR/EP having steamed/sailed through perhaps 12 hours of darkness between the evening (time?) sight and the morning (time?) sight. Traverse Tables allow this to be done on a notepad rather than drawing on the chart. But drawing lines can makes the results of any navigational calculation slips scream out at you - a major advantage. Navigators were and are taught to construct their own plotting sheets on a jotter pad to avoid the cost of bought in sheets. It's very easy to do: admittedly that rich professional Bowditch calls this an 'emergency navigation' technique. That said, there are freeware universal plotting sheets out there on the Wild Word Web.

    I've written a piece of Windows software, almanac built in, tested on XP Service Pack3, which reduces sights in three ways to provide three ways of plotting the same LOP from a single sight for comparison:
    Marq St Hilaire Intercept and Azimuth from the EP ( or AP) as is usual nowadays.
    Longitude by Chronometer and Azimuth from the nearest whole number Latitude,
    as referred to in the first paragraph.
    'Ex Meridian' Latitude and Azimuth from the nearest whole number Longitude.
    My favourite is my 'Ex Meridian' Latitude and Azimuth which gets a position line on the chart/universal plotting sheet faster than any other (for most sights). The Time Sight method comes second.

    I'm hoping to find a practising navigator on NavList who has a varied set of recorded real sights and is willing to try out the software and compare the three methods. Please get in touch by 'Reply to Author'.

    Good navigating

    Michael B
    55 North

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