MeanEquinoxOfDate
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Defined in: frame.rs:320
pub struct MeanEquinoxOfDateMean equator and equinox of date (MOD) — the equinox-based quasi-inertial
frame that drifts with precession. Belongs to the Eci category.
This is the frame the classical analytic series are actually referred to:
their mean longitudes advance at the tropical rate and their
ecliptic → equatorial rotation uses the mean obliquity of date. It is also
the equinox GMST is measured from (crate::earth::fk5::gmst1982), so a
local hour angle built as GMST + λ − α needs α to be a right ascension
in this frame.
The CIO-based counterpart is ERA + λ − α, which
Epoch::<Utc>::gmst(crate::epoch::Epoch::gmst) (a legacy name for the
Earth rotation angle) feeds; there α belongs in Cirs. A Gcrs right
ascension is not the exact partner either, but it is far closer than one in
this frame: the dominant precession-in-right-ascension term (m ≈ 4612″ per
century) is common to GMST − ERA and to α_MOD − α_GCRS and cancels,
leaving the declination-dependent n·sin α·tan δ part (≤ 0.06° in 2024)
instead of the whole 0.31°.
Differs from Gcrs by the accumulated precession — 0.335° in 2024, growing
~1.4°/century — and from the true equator of date by nutation (≤ 17″).
crate::earth::mean_equinox carries the IAU 1976 rotations to and from
Gcrs.