SLA_MAPPA - Mean to Apparent Parameters   

ACTION:

Compute star-independent parameters in preparation for conversions between mean place and geocentric apparent place. The parameters produced by this routine are required in the parallax, light deflection, aberration, and precession-nutation parts of the mean/apparent transformations. The reference frames and timescales used are IAU 1976/80.

CALL:

CALL sla_MAPPA (EQ, DATE, AMPRMS)

GIVEN:

EQ epoch of mean equinox to be used (Julian)

DATE D TDB (JD-2400000.5)

RETURNED:

AMPRMS star-independent mean-to-apparent parameters:

(1) time interval for proper motion (Julian years)

(2-4) barycentric position of the Earth (AU)

(5-7) heliocentric direction of the Earth (unit vector)

$$\times 2 /$$ (8)

(9-11) v: barycentric Earth velocity in units of c

$$\sqrt{1-\left\vert\mbox{\bf v}\right\vert^2}$$ (12)

$$3\times3$$ (13-21)

NOTES:

1.

For DATE, the distinction between the required TDB and TT is always negligible. Moreover, for all but the most critical applications UTC is adequate.

2.

The vectors AMPRMS(2-4) and AMPRMS(5-7) are referred to the mean equinox and equator of epoch EQ.

3.

The parameters produced by this routine are used by sla_MAPQK, sla_MAPQKZ and sla_AMPQK.

4.

The accuracy is limited by imperfections in the IAU 1976 and 1980 models for precession and nutation. Corrections are tabulated in IERS Bulletin B and at the present epoch are of order 50mas. An improved precession-nutation model can be introduced by first calling the present routine and then replacing the precession-nutation matrix into the AMPRMS array directly.

5.

A further limit to the accuracy of routines using the parameter array AMPRMS is imposed by the routine sla_EVP, used here to compute the Earth position and velocity by the methods of Stumpff. The maximum error in the resulting aberration corrections is about 0.3 milliarcsecond.

REFERENCES:

1.

1984 Astronomical Almanac, pp B39-B41.

2.

Lederle & Schwan, 1984. Astr.Astrophys. 134, 1-6.