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    Re: Emergency navigation: a Nocturnal as the Local Siderial Time source.
    From: Frank Reed
    Date: 2016 Sep 1, 10:24 -0700

    Tony Oz, you asked:
    "how do you deal with leap years (on the calendar scale)?"

    This is not normally useful with a nocturnal since a one-day difference is just beyond the limit of its reading accuracy. It is possible, if desired, to create a simple set of rules: apply the date offset by 6, 12, 18 hours depending on the year within the leap year cycle. Of course, one should also correct for longitude by selecting the exact date based on an estimate of UT. But once again, this is below the limit of reading accuracy for a nocturnal so it's usually ignored.

    You also asked:
    "do you take into account the difference between the Earth rotation duration agains the Sun and the stars (on the hours scale)?"

    This doesn't matter. The principle of the device handles it.

    And you asked:
    "what is the accuracy of your local time results?"

    Typically +/- "several minutes" is normal for a nocturnal.

    Finally, you asked:
    "how do you obtain UTC from it (if you do)?"

    This is impossible. There is no way to get UT from a nocturnal unless you know your longitude already (and therefore probably already know UT). The most significant use of sidereal time in modern navigation is for a Polaris correction. Sidereal time tells us the orientation of the celestial sphere relative to the observer, which lets us know where Polaris on its little circumpolar circle around the true north celestial pole. The radius of the circle is currently 40 minutes of arc, so if we ignore this correction a latitude by Polaris is rather poor. But using a nocturnal, or using a simple calculation for GHA Aries (a synonym for sidereal time, but in angular units rather than hourly units) such as is found in air almanacs and H.O. 249 etc., we can reduce that error in practice to just one or two minutes of arc.

    Frank Reed
    Conanicut Island USA

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