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    Re: Shooting my first Lunars
    From: Frank Reed
    Date: 2021 Mar 29, 21:48 -0700

    Huub Robroek,

    A week ago, you wrote:
    "Following the excellent Lunars workshop given by Frank about a month ago I started shooting lunars"

    Glad you liked it and glad to hear you're shooting! Sorry I didn't reply to you sooner. I'm also surprised that no one else replied.

    You added:
    "first by only shooting the afternoon sun-moon LD and comparing it with results from Frank's Lunar app. This worked quite well. Getting more confident I then also brought in the altitudes of moon and sun. (the quality of the mirror and its exact leveling seem to be my main accuracy-limiting factors)"

    I'm not sure I'm correctly reading what you're saying here, but remember: the altitudes of the Sun and Moon do not require high accuracy. In fact, when the lunar distance is close to 90°, the altitude of the Moon can be quite crude. Under that circumstance, +/-1° (a whole degree) is quite harnless. Under typical circumstances, the altitudes, if you choose to observe them, should be accurate to +/-6' or so. 

    By the way, for you, and for other lunarians, an easy compromise on observing the altitudes is to use an app (like mine but there are options here) for the altitudes. You shoot the actual lunar angle itself, and then you can look at an app as time permits and in a sequence of timing that would match properly observed altitudes but without having to mess with an artificial horizon and all that.

    You wrote:
    "A few days ago I got my best result until now: The Lunar app told me that my Lunar error was only 1.2' (including my altitudes sight results). With the workshop's Lunar Guidebook and notes I went through the pre-clearing and used the 3rd correction table method (which I find the easiest): The calculated GMT off-set for the cleared distance was just below 12 minutes. "

    This is puzzling. If the error in the lunar is around a minute of arc, then the error in the Greenwich time should be around two minutes of time. If it's not too much trouble, maybe you could post some of the details of your numbers. Other NavList readers get a kick out of working up lunars, too. Do you have the basic details for that lunar case? Or another?

    By the way, for others reading along, the primary method that I teach in my Lunars workshop is "Bowditch-Thomson" which is convenient and slick, and that's the "3rd correction table" that Huub is referencing here.

    You concluded:
    "But it can be neck- and hand-twisting."

    Yes, indeed! And when considering the history of these sights, I think it's important to remember that they were not urgent. One could wait until the Moon and the Sun were comfortably placed during any specific day when a navigator decided it was time to double-check the DR (in earlier decades) or check the chronometer's GMT (in later years). If it hurts, don't do it. :) 

    Great photo, by the way!!

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

       
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