A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Jan 8, 10:12 -0800
Ron. I Don’t think there’s going to much difference in the basic accuracy of the NA and AA, because they’re both churned out by the same people in the UK and the USA. As Gary says, the apparent difference is due to the way that each are presented. The AA was originally designed for fast work in a sometimes bumpy, dark aeroplane travelling at up to 10 miles a minute, with sextants which were rarely accurate in the air to more than a couple of minutes of arc, so the operating techniques, and the way the information was presented, had to be different. The air navigator didn’t have time to mess around converting arc to time. Hence the 10 minutes spacing of GHA, use of pre-computation, frequent omission of decimals of a minute of arc, and if you missed your precomputed shot time, MOB tables.
That said, I think the NA and AA paragraphs you mention are talking about slightly different things. All the AA paragraph is saying is that, with ten minute time spacing, dec and SHA of the sun and stars change so little you can ignore it for intermediate times. However, you still have to correct for the change in GHA at a rate of 15° per hour for the Sun and 15° and a touch for the stars. Most people ignore the touch.
The NA paragraph is saying that with hourly tables, you can use conversion of arc to time tables directly to work out GHA Sun and Stars, but you need to add a v correction to the mean value for the Moon and Planets, because they move less regularly. Similarly, in an hour, the declination of stars doesn’t vary, dec Sun varies slightly depending upon the time of year, Planets a bit more, and the Moon sometimes quite a bit. Hence the need for a d correction. With ten minute spacing AA users, especially if they’re using aircraft sextants accurate to only a couple of minutes of arc in any case, can thankfully forget about those particular ds and vs. DaveP
From: Ron Jones
Date: 2016 Jan 7, 20:24 -0800
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.
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."
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 ( 2◦30′.0) and GHA Aries ( 2◦30′.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."