SLA_EVP - Earth Position & Velocity   

ACTION:

Barycentric and heliocentric velocity and position of the Earth.

CALL:

CALL sla_EVP (DATE, DEQX, DVB, DPB, DVH, DPH)

GIVEN:

DATE TDB (formerly ET) as a Modified Julian Date (JD-2400000.5)

DEQX D Julian Epoch (e.g. 2000D0) of mean equator and equinox of the vectors returned. If DEQX <0, all vectors are referred to the mean equator and equinox (FK5) of date DATE.

RETURNED:

$$[\,\dot{x},\dot{y},\dot{z}\,]$$ DVB

$$[\,x,y,z\,]$$ DPB D(3)

$$[\,\dot{x},\dot{y},\dot{z}\,]$$ DVH D(3)

$$[\,x,y,z\,]$$ DPH D(3)

NOTES:

1.

This routine is used when accuracy is more important than CPU time, yet the extra complication of reading a pre-computed ephemeris is not justified. The maximum deviations from the JPL DE96 ephemeris are as follows:

• velocity (barycentric or heliocentric): 420 mm s^-1
• position (barycentric): 6900 km
• position (heliocentric): 1600 km

2.

The routine is an adaption of the BARVEL and BARCOR subroutines of P.Stumpff, which are described in Astr.Astrophys.Suppl.Ser. 41, 1-8 (1980). Most of the changes are merely cosmetic and do not affect the results at all. However, some adjustments have been made so as to give results that refer to the new (IAU 1976 `FK5') equinox and precession, although the differences these changes make relative to the results from Stumpff's original `FK4' version are smaller than the inherent accuracy of the algorithm. One minor shortcoming in the original routines that has not been corrected is that slightly better numerical accuracy could be achieved if the various polynomial evaluations were to be so arranged that the smallest terms were computed first. Note also that one of Stumpff's precession constants differs by $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.001$ from the value given in the Explanatory Supplement.