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The IAU 2000A precession-nutation theory relates the International Celestial Reference Frame to the International Terrestrial Reference Frame and has been effective since January 2003. In 2006, the IAU moved to adopt a more dynamically consistent precession model to complement the IAU 2000A nutation theory. This update – described as IAU 2006 precession in the 2009 Astronomical Almanac – is effective January 2009.
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