Reference Frames
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Astronomical Reference Frames and Earth Orientation
The hierarchy of astronomical reference frames—ICRS, GCRS, ITRS—and the Earth orientation parameters that connect them for precise position calculation.
Precession, Nutation, and the Obliquity of the Ecliptic
The physics and mathematics of precession, nutation, and the changing obliquity of the ecliptic, and their role in transforming celestial coordinates.
Apparent Positions: Aberration, Parallax, Refraction, and Light Deflection
The complete pipeline from geometric to apparent positions, covering light-time, aberration, parallax, gravitational deflection, and atmospheric refraction.