NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Nocturnals
From: Michael Dorl
Date: 2012 Jan 28, 06:25 -0600
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.