NavList:

   

Reply

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.

   

Reply