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    Re: Nocturnals
    From: Frank Reed
    Date: 2012 Jan 27, 21:57 -0800

    Sure. A nocturnal is a device for determining Local Apparent Time at night. It's accurate within about five minutes --rather rough, but better than you might expect. The orientation of the Big Dipper, or any other stars near the North Star, relative to the vertical determines the time of night. But since the Earth travels around the Sun during the course of the year, you need to adjust for the date. On one night of the year, the Big Dipper's pointer stars will be directly below the North Star at midnight. Six months later, the same stars would be directly above the North Star at midnight. There's no "equation of time" correction since we don't have to worry about the vagaries of the Sun's motion.

    Note that a nocturnal is not a "chronometer". It doesn't give you an absolute time. It gives the local time at your location, which is useful for deciding when to perform crew tasks, change shifts, etc., but not actually useful for navigation by itself. Measuring the altitude of a star at night with a sextant will also give you Local Apparent Time, if you can see the horizon. Obviously if you have such an instrument available, it beats a nocturnal for accuracy.

    -FER
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