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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2011 Mar 24, 21:58 0700
The "t" values in H.O. 218 for a zero hour angle appears to be simply the change in declination in 60 years, from 1940 to 2000, rounded to the whole minute of arc. The "t" values for other H.A.s track the change in delta d in H.O. 214 and H.O. 229. Since they are rounded, you end up with a small error after 60. Of the 22 stars, the ones most affected by this round off are Peacock, Regulus and Altair since their changes in declination are half way between the available "t" values so you can end up with a .5' error in Hc after 60 years for H.A. of zero and proportionately less for other hour angles. The easiest to deal with this slight loss of accuracy for these three stars is to multiply the "t" correction for Altair by .94; for Peacock by .96 and for Regulus by .97. If one wanted to recompute the entire tables using the 2010 stellar positions then the
existing correction table should also work for 60 more years. But if you are going to recompute the entire table then why not make the correction table more fine grained and do it for the declination change over 120 years. Personally, I think the tables are just fine the way they are, producing Hcs with practical accuracy. gl  On Thu, 3/24/11, Dave Walden <waldendand.com> wrote:
