NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2010 Mar 18, 15:44 -0400
Hi Kermit
It is a pity that they did not think to provide data to the moment. After all, the Skyscout does “know” the date, it comes from the GPS satellites. The precession calculation seems easy enough
to accomplish with the processor on board.
My limited research has indicated that the Hipparcos Catalog uses epoch 1991 (!!). This means that the positional data is 20 years out of date. Precession is clearly large enough over 20 years
to result in errors as large as, in some cases, several arc-minutes. Hence my migration to the USNO Bright Stars Catalog for exact data.
The USNO Bright Stars catalog offers data which I believe is corrected for precession, but not for proper motion. When investigating the proper motion of the stars the other day, I found this
number to be well below the ability of my sextant to resolve, in consideration of all factors. Within one year, proper motions result in relatively small corrections to the apparent distance. The highest proper motion I could find is for Bernard’s Star,
at 10 arc-seconds per year. By the 9’th star on the list, we are under 5 arc-seconds per year. For Arcturus, proper motion is about 2 arc seconds per year.
Consider, my sextant is a vernier type. It can be resolved to 10 arc-seconds but is somewhat subjective, even with the clearest of views of the vernier. When I calculate the apparent distance,
I must round to the nearest 10 arc-second graduation in order to pre-set the sextant. This can result in as much as 5 arc seconds of rounding. Surprise, proper motion is generally far lower than this! Hence I have ignored this last tweak to the apparent
distance.
Compare proper motion to the aberration, which can be as high as 20 arc-seconds, within one year. Aberration predominates. I should be able to detect this with my sextant (for the largest cases).
Ignorance of precession by Celestron is a disaster for my purpose. The data provided is simply wrong for current date. Of course, the Skyscout wasn’t intended for this purpose. It is a zero
magnification device that helps you to find stars or to identify the ones you can see. Being out by 2 or so arc-minutes just isn’t relevant for naked eye observations. Why they then offer up this in-accurate data for review to the observer is beyond me.
J
Best Regards
Brad
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