Convert star from geocentric apparent to
mean place (post IAU 1976).
CALL sla_AMP (RA, DA, DATE, EQ, RM, DM)
TDB for apparent place (JD-2400000.5)
equinox: Julian epoch of mean place
The distinction between the required TDB and TT is
always negligible. Moreover, for all but the most
critical applications UTC is adequate.
The accuracy is 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.
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.
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.