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    Re: Zero-Altitude Shot
    From: Frank Reed
    Date: 2019 Sep 8, 15:20 -0700

    "John Karl [...] talks about how to do "St. Hilaire without a Sextant".  When the upper limb of the sun drops below the horizon, you note the time and Hs as 0° 00.0', and work the sight as normal with Pub. 249."

    A few thoughts on this:

    • This has nothing to do with "St. Hilaire" (a.k.a. the "intercept method"). A celestial altitude always yields a line of position. It doesn't matter how the math is done.
    • The UL version is a bad idea. It can be quite difficult to make that "call" when the last bit of the Sun has dropped below the horizon at sunset and even more difficult to time the instant when the first bit of the Sun pops up at sunrise. So use the LL equivalent instead: time the instant when the Sun's Lower Limb is just touching the horizon. The nearly elliptical shape of the Sun can be easily extended visually even when visibility right at the horizon is a bit dodgy. In addition, if the Sun's ellipse is severely distorted, which you can see when you're doing the Lower Limb version of this sight, then you have strong evidence that the observation should not be trusted.
    • Pub. 249? Why? Use any method. Punch it up on a calculator if you like.
    • There's a tricky question glossed over in many accounts of sights like these: what should you use for the refraction? If you observe the Sun just floating on the horizon from an altitude of 100 feet, for example, what altitude do you look up in a standard refraction table (in the Nautical Almanac, e.g.)? And don't forget T/P adjustments. If the refraction is 34', then the Temperature/Pressure adjustment can easily be +/-5% or in other words, +/-1.7 nautical miles in the LOP.

    Sunrise/sunset sights definitely work, and they have been frowned upon un-necessarily in modern navigation education. But you have to be careful about refraction, and they don't work every day. If the Sun is wildly distorted as it sets, don't do this! And I will re-iterate the most important recommendation: use the Sun's Lower Limb. As the Sun is setting, mark the time just as the lower edge reaches the horizon. And although you don't need significant sextant skills to do a sight like this, it does help to have a sextant available both for the magnification of the scope and also for the shades since the rising or setting Sun can sometimes be uncomfortably bright. If you try this, set the sextant to 0° 00.0' (assuming zero index error, which should always be zero-ed out in any case). Then swing in a medium-density horizon shade. Use the most poweful scope available. And then just look through the sextant. Don't touch the micrometer... Just sit back and wait until the lower edge of the Sun (or the visually-extended arc of the Sun's ellipse) touches the horizon. Note the UT, and you're done. 

    Frank Reed

       
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