heyoka.model.iau2006

heyoka.model.iau2006(time_expr: expression = heyoka.time, thresh: float = 1e-6) → list[expression]

IAU2000/2006 precession-nutation theory.

Added in version 7.3.0.

This function will return a set of three expressions representing the \(X\), \(Y\) and \(s\) angles from the IAU2000/2006 precession-nutation theory as a function of the input time expression time_expr.

\(X\) and \(Y\) are the celestial intermediate pole (CIP) coordinates in the International Celestial Reference System (ICRS). They describe the position of the CIP relative to the Geocentric Celestial Reference Frame (GCRF), which is aligned with the ICRS. \(s\) is the Celestial Intermediate Origin (CIO) locator. It represents an additional rotation in the transformation between celestial and terrestrial reference frames. For more information about these quantities, please consult standard references such as Vallado’s “Fundamentals of Astrodynamics” (Chapter 3) and Chapter 5 of the IERS conventions.

The angles are returned in radians. time_expr is expected to represent the number of Julian centuries elapsed since the epoch of J2000 in the terrestrial time scale (TT).

thresh represents the truncation threshold: trigonometric terms in the theory whose coefficients are less than thresh in magnitude will be discarded. In order to formulate the full theory without truncation, use a thresh value of zero.

Note

A tutorial is available showcasing the use of this function, including accuracy comparisons for several values of the thresh argument.

Parameters:

• time_expr – the input time expression.

• thresh – the truncation threshold for the coefficients of the trigonometric series (in arcseconds).

Returns:

expressions for the \(X\), \(Y\) and \(s\) angles from the IAU2000/2006 precession-nutation theory.

Raises:

ValueError – it thresh is negative or non-finite.