# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Yet another 2102-D modification (summary)
From: Greg Rudzinski
Date: 2013 Jan 20, 16:17 -0800

John,

Just mark in your equation of time values on the disc. 1 degree equals 4 minutes. Put down the closest template lined up allowing for the equation of time then plot the Sun at the correct LAN altitude for the days declination. I also draw a larger circle around the plotted Sun so that the tangent of the template's zero altitude gives the civil twilight view of the stars.

Greg Rudzinski

[NavList] Re: Yet another 2102-D modification (summary)
From: John Forrest
Date: 20 Jan 2013 14:21
I've been experimenting a bit with my easystarfinder approach - as in i.e. easystarfinder.blogspot.co.uk, although I've yet to write this up. The main changes I tried were as follows:

1. Emphasise Noon GHA(Aries) rather than Midnight GHA(Aries) - so work out a "NoonPos" of the GHA(Aries) at GMT Noon for the day in question and subtract West latitute or add East latitude for that. Put the 12:00 position on the hour disk at that point. In theory this is a neutral change.

2. Mark the GHA (Aries) at Noon values for the 1st and 15th of each month (rounded to nearest degree) on the base. This then replaces the RoughnReady almanac.

3. Draw the ecliptic on the base. I tried to follow Gary La Pook's instructions, but found it hard to do it very accurately. It took me several goes and slightly different instructions. [In the end, I marked both the summer and winter solstice points and drew the circle whose centre was on a straightline between 90 and 270 and which went through both soltice points.]

The question was whether I could just use the rough Sun position that came from drawing a straightline between the GHA(Aries) Noon position and the centre, and use it to predict twilight without too much extra. If there was no effect of the equation of time this would be the position of the sun.

To be honest I got very mixed results. Not allowing for the equation of time means that the sun can be a few degrees the wrong way.

However even if I plot the correct place of the Sun based on its SHA for the day in question, the results are mixed. I think this is down to a few issues:
1) The angle of the horizon to the body is very shallow, so saying when it has crossed is not very easy to say. All the inaccuracies of the 2102-D come to the fore.
2) We get back to the central problem that I'm at 51.5N but I have to pretend to be 55N.
3) I'm not sure how to model the sun on the starfinder. The sun is large enough that in sights we'd be interested in top and bottom rim but on the starfinder I'm still moddeling it like a star or a planet, as just a dot.

I still think it worth drawing the ecliptic since any planets or the sun when placed properly should be at or near the ecliptic and it acts as a check - if you try to plot SHA instead of RA (for example) it is readily obvious. However, even with the equation of time being allowed for (as Gary does) I'd wonder how accurate the twilight/dawn predictions were.

[Byron has the luxury of placing the zenith point directly which I think solves some of the issues but even then I'm curious as to the prediction since no allowence is made for the equation of time, as far as I can see.]

QUESTION: has anyone tried modelling the sun on the starfinder as a circle rather than a point, and does it work??

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