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    Re: Precession and Nutation
    From: Paul Hirose
    Date: 2017 Feb 01, 14:00 -0800

    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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