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Michael Dorl writes of "estimating the departure of Polaris from 90
degrees".

The circumpolarity of Polaris is small in our time but was very different in
the time of Columbus.  Mid-16th century circumpolarity was 3 degrees 27
minutes.  This had to be factored into observations of Polaris made to
facilitate the practice of 'running the latitude down'.  There were many
ingenious mnemonic diagrams giving the altitude of the pole star at
successive positions in its apparent rotation around the true pole.  These
have the collective name "Rosa das alturas do norte".

According to Bedini et. al., the circumpolarity of Polaris was not know
prior to Columbus's crossing.  Columbus discovered the Polaris was
circumpolar  through the practice of comparing Polaris's bearing with
magnetic north as given by the mariner's compass.  Thus he was alerted to
the unreliability of a single reading of of Polaris's altitude.  Taking
readings, as was the practice, at dawn and dusk, provided a set of altitude
and azimuth differences between Polaris and the compass magnetic north that
could be averaged.  Even though Polaris had a circumpolarity of 3.5 degrees,
averaging reduced the error in determining altitude and azimuth to 0.75
degrees or less.

Using a nocturnal in our own times does not require a correction for
circumpolarity because precession has reduced its value to less than the
accuracy of the instrument.   If the instrument that Michael has made has
such a scale, it may be 350 years out of date!

I have previously posted on the use of a nocturnal to find the time of high
water, given the establishment of the port.

Ref: Bedini S. A. (Ed) 1998. "Christopher Columbus and the age of
exploration, an encyclopedia"  N.Y. Da Capo Press.


----- Original Message -----
From: "Michael Dorl" 
To: 
Sent: Saturday, January 28, 2012 12:25 PM
Subject: [NavList] Re: Nocturnals


> 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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