NavList:

"Byron: I don't see that you can better my 2 minute star finder.
It's designed to be used by a 5-6th greader."

Byron, my original aim at adding my Hours Disk to the 2102-D was to just make the thing I could actually buy (i.e. the 2102-D) easier to use. Having done so I think the two approaches have similar complexities in terms of setup, at least - contrast one addition and one subtraction (of whole degrees) with adding two times. That is not to say the 2-minute is perhaps more intuitive in some ways: moving the horizon for the correct latitude and even having the stars on top (so the stars seem to move and not the earth) gives you this.

However there are some advantages of the Hour Disk, not that I was expecting advantages when I set out to do this. The times on the disk are very easy to read. Once setup the times are directly read off the Hours Disk. If you want to see the state of the sky at a particular time, you just point the pointer on the standard 2102-D horizon disk to that time. If you want to see when (say) a particular body is going to be south of you, you literally move the horizon disk so that the 180 degree bearing line is above the body concerned and then read off the time that the pointer is pointing to. No need for additional straight edges. No need for additional sums.

Where your approach scores most is the extra accuracy that comes from placing the Zenith at the correct place, even if the horizon is not quite correct (ie. for 51N using 55N but placing it as if 51N). This ability comes partly from having a separate,small horizon disk - compared to 2102-D.

However there is something I don't understand about this: does this not make the horizon disk "single use", in that once you have a hole in it, it is not going to be much use at a neighbouring latitude? What would happen in the context of a passage or even as a training aid if the user took a trip somewhere? Does this mean that for real you'd need lots of horizon disks, and at that point would it not be better to actually have then with the correct horizon anyway?

John

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