181 lines
7.3 KiB
Rust
181 lines
7.3 KiB
Rust
//! The IAU 2006 precession model and the frame bias.
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//!
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//! Precession is the slow, secular motion of the Earth's rotation axis about
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//! the pole of the ecliptic, caused mainly by the torque of the Moon and the
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//! Sun on the equatorial bulge. The IAU 2006 model expresses it as a
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//! rotation from the celestial reference frame to the mean equator and
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//! equinox of date, built from four angles.
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//!
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//! The frame bias accounts for the small offset between the orientation of
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//! the International Celestial Reference System and the dynamical mean
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//! equator and equinox of J2000.0. It is a constant rotation, and omitting
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//! it places a position about twenty milliarcseconds away, which is well
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//! above the precision of a modern transformation.
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//!
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//! Source: the P03 precession theory of Capitaine, Wallace and Chapront,
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//! adopted by the International Astronomical Union in 2006, as implemented
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//! in the IAU SOFA library and its ERFA derivative, functions `eraPfw06`,
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//! `eraPmat06`, `eraFw2m`, `eraBi00` and `eraObl06`.
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use apsis_math::Mat3;
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use core::f64::consts::PI;
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/// Converts arcseconds to radians.
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const ARCSECOND_TO_RADIAN: f64 = PI / (180.0 * 3_600.0);
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/// The frame bias in right ascension, in radians.
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///
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/// The offset between the origin of the International Celestial Reference
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/// System and the dynamical equinox of J2000.0. The value is
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/// minus 0.0146 arcseconds.
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pub const FRAME_BIAS_RIGHT_ASCENSION: f64 = -0.014_6 * ARCSECOND_TO_RADIAN;
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/// The frame bias in declination, in radians.
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///
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/// The offset between the pole of the International Celestial Reference
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/// System and the mean pole of J2000.0. The value is minus 0.0068192
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/// arcseconds.
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pub const FRAME_BIAS_DECLINATION: f64 = -0.006_819_2 * ARCSECOND_TO_RADIAN;
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/// The frame bias in longitude, in radians.
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///
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/// The offset between the two origins measured along the equator. The value
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/// is minus 0.041775 arcseconds.
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pub const FRAME_BIAS_LONGITUDE: f64 = -0.041_775 * ARCSECOND_TO_RADIAN;
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/// The four angles that describe the precession from J2000.0 to a date.
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///
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/// They are applied in the order the reference uses: a rotation by
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/// `gamma_bar` about the third axis, then by `phi_bar` about the first, then
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/// by minus `psi_bar` about the third, then by minus `epsilon_a` about the
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/// first.
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#[derive(Debug, Clone, Copy, PartialEq)]
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pub struct PrecessionAngles {
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/// The precession of the equator along the ecliptic, in radians.
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pub gamma_bar: f64,
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/// The inclination of the moving equator to the ecliptic, in radians.
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pub phi_bar: f64,
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/// The accumulated precession in longitude, in radians.
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pub psi_bar: f64,
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/// The mean obliquity at the date, in radians.
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pub epsilon: f64,
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}
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/// Returns the mean obliquity of the ecliptic, in radians.
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///
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/// The polynomial is the IAU 2006 expression, evaluated for a time in Julian
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/// centuries from J2000.0 on the Terrestrial Time scale. The constant term
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/// is 84381.406 arcseconds, which is the obliquity at J2000.0.
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///
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/// Source: the IAU 2006 precession, as implemented by `eraObl06`.
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pub fn mean_obliquity(t: f64) -> f64 {
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let arcseconds = 84_381.406
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+ t * (-46.836_769
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+ t * (-0.000_183_1
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+ t * (0.002_003_40 + t * (-0.000_000_576 + t * (-0.000_000_043_4)))));
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arcseconds * ARCSECOND_TO_RADIAN
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}
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/// Returns the four precession angles for a time.
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///
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/// The time is in Julian centuries from J2000.0 on the Terrestrial Time
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/// scale.
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///
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/// Source: the P03 precession, as implemented by `eraPfw06`.
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pub fn precession_angles(t: f64) -> PrecessionAngles {
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let gamma_bar = -0.052_928
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+ t * (10.556_378
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+ t * (0.493_204_4
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+ t * (-0.000_312_38 + t * (-0.000_002_788 + t * (0.000_000_026_0)))));
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let phi_bar = 84_381.412_819
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+ t * (-46.811_016
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+ t * (0.051_126_8
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+ t * (0.000_532_89 + t * (-0.000_000_440 + t * (-0.000_000_017_6)))));
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let psi_bar = -0.041_775
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+ t * (5_038.481_484
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+ t * (1.558_417_5
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+ t * (-0.000_185_22 + t * (-0.000_026_452 + t * (-0.000_000_014_8)))));
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PrecessionAngles {
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gamma_bar: gamma_bar * ARCSECOND_TO_RADIAN,
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phi_bar: phi_bar * ARCSECOND_TO_RADIAN,
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psi_bar: psi_bar * ARCSECOND_TO_RADIAN,
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epsilon: mean_obliquity(t),
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}
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}
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/// Returns the frame bias rotation matrix.
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///
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/// The bias is a constant rotation from the International Celestial
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/// Reference System to the dynamical mean equator and equinox of J2000.0.
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/// Its three angles are far below a tenth of an arcsecond, but omitting it
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/// would leave a systematic error that is larger than the precision of the
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/// nutation series.
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///
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/// Source: as implemented by `eraBi00`, which gives the bias as three small
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/// rotations rather than as a matrix.
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pub fn frame_bias_matrix() -> Mat3 {
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// The reference builds the bias from three elementary rotations, and
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// the result is reproduced here in the same order.
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rotation_z(FRAME_BIAS_LONGITUDE)
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* rotation_x(FRAME_BIAS_DECLINATION)
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* rotation_y(FRAME_BIAS_RIGHT_ASCENSION)
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}
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/// Returns the rotation from the celestial reference frame to the mean
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/// equator and equinox of date.
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///
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/// This is the bias-precession matrix. Nutation is applied on top of it to
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/// reach the true equator and equinox of date.
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///
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/// The time is in Julian centuries from J2000.0 on the Terrestrial Time
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/// scale.
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///
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/// Source: as implemented by `eraPmat06`.
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pub fn bias_precession_matrix(t: f64) -> Mat3 {
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let angles = precession_angles(t);
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frame_to_mean_of_date(&angles)
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}
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/// Returns the rotation from the celestial reference frame to the mean
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/// equator and equinox of date, given precomputed precession angles.
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///
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/// Source: as implemented by `eraFw2m`, which applies the four elementary
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/// rotations in sequence. The reference helper composes each new rotation
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/// on the left of the accumulator, so the product here is written with the
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/// last rotation applied first, in the same order.
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pub fn frame_to_mean_of_date(angles: &PrecessionAngles) -> Mat3 {
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rotation_x(-angles.epsilon)
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* rotation_z(-angles.psi_bar)
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* rotation_x(angles.phi_bar)
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* rotation_z(angles.gamma_bar)
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}
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/// Returns a rotation of `angle` about the first axis, in radians.
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///
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/// The convention matches the reference helpers `eraRx` and `eraRz`: the
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/// rotation is applied to the frame rather than to a vector, so the matrix
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/// is the transpose of the one that would rotate a vector by the same angle.
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/// Composing `a * b` therefore applies `b` first, and the products in this
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/// module read in the order the rotations are physically applied.
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pub(crate) fn rotation_x(angle: f64) -> Mat3 {
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let (sine, cosine) = angle.sin_cos();
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Mat3::from_rows([1.0, 0.0, 0.0], [0.0, cosine, sine], [0.0, -sine, cosine])
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}
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/// Returns a rotation of `angle` about the second axis, in radians.
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///
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/// The sign convention matches the other axis rotations in this module.
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fn rotation_y(angle: f64) -> Mat3 {
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let (sine, cosine) = angle.sin_cos();
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Mat3::from_rows([cosine, 0.0, -sine], [0.0, 1.0, 0.0], [sine, 0.0, cosine])
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}
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/// Returns a rotation of `angle` about the third axis, in radians.
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///
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/// The sign convention matches the other axis rotations in this module.
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pub fn rotation_z(angle: f64) -> Mat3 {
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let (sine, cosine) = angle.sin_cos();
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Mat3::from_rows([cosine, sine, 0.0], [-sine, cosine, 0.0], [0.0, 0.0, 1.0])
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}
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