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Module precession

celestial_core

Module precession 

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Precession models for transforming coordinates between epochs.

Precession is the slow, continuous change in the orientation of Earth’s rotational axis caused by gravitational torques from the Sun and Moon acting on Earth’s equatorial bulge. The axis traces a cone in space with a period of approximately 26,000 years (the “Platonic year”), causing the celestial pole to drift among the stars and the vernal equinox to move westward along the ecliptic.

This module provides rotation matrices that transform celestial coordinates from one epoch to another, accounting for the accumulated precession between epochs.

§Available Models

§IAU 2000

The IAU 2000 precession model (PrecessionIAU2000) uses the Lieske (1977) precession angles with corrections from the IAU 2000A nutation model. It computes three rotation angles (psi_A, omega_A, chi_A) to construct the precession matrix.

The frame bias matrix accounts for the offset between the J2000.0 dynamical frame and the ICRS (International Celestial Reference System), which amounts to a few milliarcseconds.

§IAU 2006

The IAU 2006 precession model (PrecessionIAU2006) uses the Fukushima-Williams four-angle formulation (gamma_bar, phi_bar, psi_bar, epsilon_A). This parameterization provides improved numerical stability and separates the frame bias from the precession proper.

The Fukushima-Williams angles represent:

  • gamma_bar: Frame bias in right ascension
  • phi_bar: Obliquity of the ecliptic at J2000.0
  • psi_bar: Precession in longitude
  • epsilon_A: Mean obliquity of date

IAU 2006 is the current standard and should be preferred for new applications.

§Output Matrices

Both models produce a PrecessionResult containing:

  • bias_matrix: Transforms from GCRS (Geocentric Celestial Reference System) to the mean equator and equinox of J2000.0. This is constant for a given model.

  • precession_matrix: Transforms from the mean equator and equinox of J2000.0 to the mean equator and equinox of date. At J2000.0 (t=0), this is the identity.

  • bias_precession_matrix: The combined transformation from GCRS to the mean equator and equinox of date (bias_precession = precession * bias for IAU 2000, computed directly via Fukushima-Williams for IAU 2006).

§Time Argument

IAU 2000 takes time as Julian centuries of TT (Terrestrial Time) since J2000.0. IAU 2006 takes a two-part Julian Date in TT for improved numerical precision.

Re-exports§

pub use iau2000::PrecessionIAU2000;
pub use iau2006::PrecessionIAU2006;
pub use types::BiasMatrix;
pub use types::PrecessionMatrix;
pub use types::PrecessionResult;

Modules§

iau2000
IAU 2000 precession model.
iau2006
IAU 2006 precession model.
types
Types for representing precession computation results.

Traits§

PrecessionCalculator
Trait for types that can compute precession matrices.