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    Re: Time sight primer
    From: David C
    Date: 2020 Oct 10, 19:41 -0700

    In old textbooks there is a big emphasis on the times equations. Without having studied them in detail and learned them by rote they can be confusing. Here is an example.

    A week or so ago I took some PM sights and the next day some AM sights. I decided to work a sight from each day as a time sight, reduce them with Martelli which I was studying, and plot  the  LOP using the two point Sumner. I would use a plane chart. Easy I though. I could start a thread called "mix and match"  - at each point in the process I could chose a different path. For no particular reason I worked the PM sight first. The LOP looked sensible and passed close to my GNSS position.

    When I worked the AM sight things turned to custard. The resulting longitude did not make sense. I finally realised that the culprit was

    ATS = HATS + 12

    ATS  Apparant time at ship     HATS hour angle true sun.

    Martelli gives the ATS and  uses AM/PM notation. In the afternoon the "PM" takes care of the 12 hour factor.  Conversion from ATS to t is a simple matter of using the time to arc table in Norie.  However in the forenoon "AM" does not include 12 so the simple table lookup does not work.

    I hope that makes sense. I had just resolved the matter when I read Frank's post but will not know for certain until that I am correct until  I plot the second LOP and confirm that it passes near my GNSS position

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