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    Re: Evaluation of H.O. 218
    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.


    --- On Thu, 3/24/11, Dave Walden <waldendand@yahoo.com> wrote:

    From: Dave Walden <waldendand@yahoo.com>
    Subject: [NavList] Re: Evaluation of H.O. 218
    To: NavList@fer3.com
    Date: Thursday, March 24, 2011, 9:00 PM

    Attached are two spread sheets to generate HO218 tables. One does the star tables in the front. (In fact for any of the 57 navigations stars + Polaris). The other does the Dec tables from the back. Refraction is calculated using a two term tan ZD + tan ZD ^3 equation. The two coefficients are based on temperature pressure values from the 'standard' atmosphere at 5000 feet. (Method given in the spread sheet.) The Dec table seems to agree exactly down to altitudes of about 5 degrees or so, where it misses by 1. (I do only have 1 vol of 218, so limited checking.)

    The star tables take some more work, and I'm still playing. "t" values are calculated. (A version of the t interpolation table Gary gave is also included.) I seem to miss by 1 min every 10 or so. It seems 218 uses star data (RA, Dec, and PM) from the Boss General Catalog of 1937. I used modern values. I used modern precession values too. Since 218 is linear in time, it neglects nutation and annual aberration. Seems reasonable for "air" accuracy, since both are small with virtually no secular component.

    Interestingly, in using 214 to confirm, I find about 9 differences of .1 min in a single Dec column. (see spread sheet)

    ((Warning, an effort to amuse and entertain myself. Errors likely remain.))

    >I haven't had time to read all your posts on these tables, but since you've given >them a thorough review, could you rate them? Are they a B+ or an A, as tables go? >I haven't seen a complete set of them ever. Are they good enough for us to >consider re-generating them (easy enough today) and making them available for >small boat navigators and other navigation enthusiasts?
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