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    Re: Nocturnals
    From: Michael Dorl
    Date: 2012 Jan 28, 06:25 -0600

    On 1/27/2012 11:57 PM, Frank Reed wrote:
    > 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
    I beleive they were also used to estimate local tide conditions.
    The one I made also has scales on the back for estimating the departure
    of Polaris from 90 degrees.

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