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Yet another 2102-D modification (summary)
From: John Forrest
Date: 2013 Jan 12, 12:11 -0800

As a follow up, I thought I'd write a summary of the mail thread from "Yet another 2102-D modification", Basically there are four separate proposals:

1. My (John Forrest's) proposal

See: http://easystarfinder.blogspot.co.uk

Introduce a new "Hours Disk". Set midnight position based on GHA(Aries) and longitude. Time works in GMT, and can literally read times off the scale. Need an almanac of sorts, but there is a summary of average GHA(Aries) as a separate table.

2. Greg Rudzinski's

See: http://gregrudzinski.blogspot.co.uk/2010/10/set-2102-d-star-finder-without-nautical.html

Use a scaled and marked up radar plotting sheet to setup the starfinder - you place the starfinder over the sheet to set things up, so effectively there are extra scales but only during setup. Time used is Zone Time.

3. Gary la Pook's

There are a number of postings, but the most informative was I think:

http://www.fer3.com/arc/m2.aspx?i=107982&y=200904

Gary has the most complex updates to the 2102-D, having added a number of extra features and also making use of the red scale. Presumably this is for setup only. Basically this proposal adds back many of the features of an astrolabe: the ecliptic; time scales (hours on blue disks and minutes on the red); dates; the "analemas" diagrams to relate local solar and local mean times; the twilight altitude line 12 degrees below the horizon.

This is all quite complex. However, the effect is to not only put the sky in the correct position without an almanac, but also the position of the sun. This in turn allows nautical twilight times to be predicted without an almanac. It is going to be a bit out, though, because it makes no allowance for the differences year on year. Any times are going to be LMT.

4. Byron Franklin's

Last but by no means least. Byron's is the most different because he proposes a new starfinder rather than a mod to the 2102-D. Byron's also adds average almanac info to the starfinder, so again the sky setup can be done without reference to an almanac. Byron uses times throughout the scheme - adding the GMT of the proposed sight time to the GHA(Aries) value read off as an equivalent time rather than as degrees.

A key feature of Byron's starfinder is that the horizon is a separate disk that can be placed at the correct equivalent longitude and if required taped there. Also it can be placed so that the Zenith point is correct, even if the correct horizon disk for the latitude is not available. This potentially gives more accurate results. I'm still slightly puzzled as what this would mean to the lifetime of this design if it were ever commercially produced.

Although Byron's design virtually works throughout in times, it is not possible to read the times of events directly. You always have to do a sum to convert from GMT to the times shown on the device or vice versa.

There we have it. Some have suggested you merely need to maintain a plotted position of the sun and work out the time from that. I can't see how that would not be merely rough.

Now the more I look at this, the more I prefer my own mod. The ability to directly work in GMT seems a major advantage. Of course without Byron's flexible horizon, all the direct 2102-D suffer from a problem where they only quote reliably if the latitude value ends in "5" - 5, 15, 25 etc.

Whatever, I'd recommend anyone who wants to use a 2102-D to check these proposals out. They each have their merits.

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