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On 2017-01-31 7:13, Mike Freeman wrote:
> Initially I became interested in Precession and Nutation because I assumed, 
possibly incorrectly that the corrections in Polaris tables in NA and the 
corrections in AP3270 Vol 1 Table 5 were both for precession and nutation, 
although this is only made clear in Polaris tables. However the reason I 
became confused is entering argument for Polaris tables is MONTH and entering 
argument for AP3270 vol 1 Table 5 is YEAR.

The almanac Polaris table includes aberration, which causes the apparent
place of all stars to oscillate a few tenths of a minute on an annual
cycle. Therefore the correction varies month by month. If we compute the
apparent SHA and declination of Polaris with respect to a fixed
coordinate system (say, the equator and equinox at the beginning of
2017), it's easy to see aberration at work:

apparent place, 2017 Jan 1 equator
316°22.7'  89°20.3' Jan 2017
316°50.9'  89°20.3' Mar
317°07.0'  89°20.0' May
316°55.6'  89°19.7' Jul
316°28.1'  89°19.7' Sep
316°11.1'  89°20.0' Nov
316°22.6'  89°20.3' Jan 2018

Note how Polaris returns to its original place after one year. (The high
declination exaggerates the SHA variation. In great circle terms it's
almost 100 times less.)

By contrast, the geometric place of Polaris with respect to the equator
of date shows the effects of precession and nutation without
contamination from aberration:

geometric place, equator of date
316°39.3'  89°20.0' Jan 2017
316°37.6'  89°20.1' Mar
316°35.0'  89°20.1' May
316°31.1'  89°20.1' Jul
316°29.0'  89°20.2' Sep
316°27.0'  89°20.2' Nov
316°22.5'  89°20.3' Jan 2018

Here the effect is not periodic but secular, i.e., the coordinates move
steadily away from the original position. Actually, precession is
periodic, but its 26,000 year cycle makes it seem secular on
navigational time scales.


> What is not clear is whether any element of P & N is taken into account when 
calculating GHA Aries. At the moment I am thinking this is why entry 
arguments for Polaris tables and AP3270 vol1 Table5 differ in respect of 
MONTH or YEAR.

The Nautical Almanac tables include precession, nutation, and (when
applicable) aberration. That's practical because the book only covers
one year. On the other hand, a book valid for several years can use
fixed star positions plus a correction for precession and nutation
applied by the user. In that case the aberration correction (with its
dependence on time of the year) can be omitted since it's never more
than a few tenths of a minute.

   

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