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    Re: Pub. 249 vol1
    From: Frank Reed
    Date: 2014 Jan 1, 11:07 -0800

    One can easily double-check values in Pub. 249 vol. 1 using the web calculation app that we all know and love on the US Naval Observatory web site: the "Celestial Navigation Data for Assumed Position and Time" located at http://aa.usno.navy.mil/data/docs/celnavtable.php.

    This calculation engine is exactly what was used to generate the tables in the current 2015 edition of Pub. 249. The tables of Pub. 249 vol. 1 are literally CONTAINED WITHIN this USNO software product. The web page does not have an entry for "LHA Aries" however the results page displays GHA Aries along with the other celestial data. So pick an easy date (today is fine). Set the latitude to the tabulated latitude, in the case at hand 35°00.0'N, and set the longitude to 0°00.0. Then adjust the UT as entered until it produces a GHA Aries equal to your required LHA Aries (since GHA Aries is identical to LHA Aries when you're on the Prime Meridian). In this case if you set the UT to 19:34:10, you get the requested LHA Aries = 29°. And from the results page we find that the altitude and true azimuth of Capella are Hc=+50° 48.2' and Zn=57.9°. Of course, this matches the values tabulated in the current edition which are 50°48' and 058°.

    Site reduction is sky simulation. We simulate the appearance of the sky from some known location and then compare what we measure against that simulation (with the observed altitudes corrected or not, depending on whether the simulation is geocentric or local). The differences between the calculated altitudes in the simulation and the observed altitudes are the "intercepts". Note that the coarseness of the degree grids in Pub.249 v.1 are necessary in a portable volume. But if you have access to this USNO web page, you can, of course, set the known location (assumed position) to match your known coordinates. Then the intercepts are simply the residual errors in your observations. In effect, the web-based calculation app is a "dialable" version of Pub. 249.

    Finally, note that the calculation for a given date using the USNO web tool includes aberration and precession and even proper motion of the stars, so if you want to compare against a printed volume for a given year, maybe Pub.249 from 1960, then you need to set the date correctly and possibly average over the year centered on that epoch date. Otherwise you will find small discrepancies in recent years and increasingly large differences in earlier decades.


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