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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Accuracy of Nautical Almanac vs Air Almanac
From: Gary LaPook
Date: 2016 Jan 8, 07:57 +0000
That's because the values are tabulated every ten minutes instead of only once an hour. You must interpolate between those ten minute tabulations UNLESS you use the air navigation method of taking your sight at pre-planned times, exactly on the ten minute mark, not hard to do this.

gl

To: garylapook@pacbell.net
Sent: Thursday, January 7, 2016 8:39 PM
Subject: [NavList] Accuracy of Nautical Almanac vs Air Almanac

When checking data for the Sun's GHA at UT hour intervals the Nautical Almanac values differ from those in the Air Almanac by either 0.1' or 0.2' based on the extracts below it appears that the Air Almanac values are closer to the correct value.
Ron Jones
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See Page 261 of the Nautical Almanac for a note on Accuracy.
"The quantities tabulated in the Nautical Almanac are generally correct to the nearest 0'.1; the exception is the sun's GHA which is deliberately adjusted by up to 0'.15 to reduce the error due to ignoring the v-correction.  The total GHA and Dec at intermediate times cannot be obtained to this precision, since at least two quantities must be added;  moreover, the v - and d -corrections are based on mean values of v and d and are taken from tables for the whole minute only.  The largest error that can occur in the GHA or Dec of any body other than the Sun or Moon is less than 0'.2; it may reach 0'.25 for the GHA of the Sun and 0'.3 for that of the Moon."
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Air Almanac Page A4
"8. General. When an accuracy of better than one minute of arc (1) is required, as for
example when using some form of astro-tracker, the values tabulated in The Air Almanac enable the GHA and Dec. of the Sun and stars to be taken out with a nominal accuracy of 0.1.  The values of the GHA and Dec. of the Sun and GHA Aries are taken from the daily pages, in the usual way, for the 10m immediately prior to the time of observation. No interpolation is required for the Sun’s Dec., but increments must be added to the GHA of the Sun and Aries for the excess of the time of observation over the tabular 10m. These increments must be taken from the interpolation tables on the inside front cover and the flap. These interpolation tables are based on mean values of the increments of GHA Sun ( 230.0) and GHA Aries ( 230.4) in 10m.  Separate tabulations of the SHA and Dec., to a precision of 0.1, of 173 stars are given on pages A158–A163. The values are tabulated for the middle of each month and no interpolation is necessary. GHA is found, as usual, by adding the SHA to GHA Aries."

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