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    Re: Longitude by Time Sight...good enough at sea?
    From: Brad Morris
    Date: 2012 Nov 21, 17:15 -0500

    Hi Doug

    Just my two cents.

    There is no reason why not to use the time sight!  We know that Worsley used the time sight for navigation with Shackleton, less than 100 years ago,albeit with latitude determined otherwise.  It works!

    The method primarily used the sun for latitude, at midday with the longitude at early AM or late PM.  Thus every lat lon fix is essentially an advancing of these particular two LOPs to each other.  We advance the morning longitude to the midday latitude.  Similarly, in the evening, we advance the latitude to the evenings longitude.  The weakness is in the assumed change to latitude or longitude as the boat sails.  At slow speeds, I don't feel that this is a major issue.  Of course, that's my opinion, others may differ.

    The method may lack the "precision" offered by a fix done by modern methods.  The modern method normally only advances LOPs less than hour (for a round of sights), so presumably, its more accurate.

    In my view, this is a "you pick" choice.  Both are functional.

    Regards
    Brad

    On Nov 21, 2012 4:52 PM, "Doug MacPherson" <dwmacpherson2000@gmail.com> wrote:

    All:

    I recently was reading the posts on 10 nm accuracy being good enough when fixing your position at sea, far from any land mass.

    I recently taught my self the Time Sight method of computing Longitude (using an assumed latitude) from an 1938 copy of Bowditch using the haversine tables found in Table 34 of Bowditch. This seemed to be the normal procedure prior to the distribution of Ho 229 and others.

    Not a lot of computation involved and you can plot your fix directly onto the chart.
    I consistently get within 5 NM of my actual position when shooting from land..all be it, with the correct Latitude, but varying the Latitude by a few nm keeps me within the 10 nm.

    Any reason why traditional sailors travelling the seas would not use this method when far from land today when they are inclined to turn off the GPS?

    In Northern climes....Get your lat. from Polaris in the morning....Take a time sight just after the sun has risen (best accuracy when sun is near the prime vertical) using you assumed Polaris Lat carried forward.....Get your lat. at noon from the sun...and then another Long. prior to the sun setting with your assumed Lat worked from your noon sight.


    Would seem to save a lot of plotting.

    Thoughts?

    Doug
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