SLA_AMP

Apparent to Mean

ACTION:: Convert star $[α, δ]$ from geocentric apparent to mean place (post IAU 1976).
CALL:: CALL sla_AMP (RA, DA, DATE, EQ, RM, DM)

RA,DA	D	apparent $[α, δ]$ (radians)

DATE	D	TDB for apparent place (JD $-$ 2400000.5)

EQ	D	equinox: Julian epoch of mean place

RM,DM	D	mean $[α, δ]$ (radians)

NOTES:

(1): The distinction between the required TDB and TT is always negligible. Moreover, for all but the most critical applications UTC is adequate.
(2): Iterative techniques are used for the aberration and light deflection corrections so that the routines sla_AMP (or sla_AMPQK) and sla_MAP (or sla_MAPQK) are accurate inverses; even at the edge of the Sun’s disc the discrepancy is only about 1 nanoarcsecond.
(3): Where multiple apparent places are to be converted to mean places, for a fixed date and equinox, it is more efficient to use the sla_MAPPA routine to compute the required parameters once, followed by one call to sla_AMPQK per star.
(4): For EQ=2000D0, the agreement with ICRS sub-mas, limited by the precession-nutation model (IAU 1976 precession, Shirai & Fukushima 2001 forced nutation and precession corrections).
(5): The accuracy is further limited by the routine sla_EVP, called by sla_MAPPA, which computes the Earth position and velocity using the methods of Stumpff. The maximum error is about 0.3 milliarcsecond.

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