Processing math: 100%

SLA_EPV

Earth Position & Velocity (high accuracy)

ACTION:
Earth position and velocity, heliocentric and barycentric, with respect to the Barycentric Celestial Reference System.
CALL:
CALL sla_EPV (DATE, PH, VH, PB, VB)
GIVEN:

DATE

D

TDB Modified Julian Date (Note 1)


RETURNED:

PH

D(3)

heliocentric [x,y,z], AU

VH

D(3)

heliocentric [ẋ,ẏ,ż], AU d−1

PB

D(3)

barycentric [x,y,z], AU

VB

D(3)

barycentric [ẋ,ẏ,ż], AU d−1


NOTES:
(1)
The date is TDB as MJD (=JD−2400000.5). TT can be used instead of TDB in most applications.
(2)
The vectors are with respect to the Barycentric Celestial Reference System (BCRS). Positions are in AU; velocities are in AU per TDB day.
(3)
The routine is a simplified solution from the planetary theory VSOP2000 (X. Moisson, P. Bretagnon, 2001, Celes. Mechanics & Dyn. Astron., 80, 3/4, 205-213) and is an adaptation of original Fortran code supplied by P. Bretagnon (private communication, 2000).
(4)
Comparisons over the time span 1900-2100 with this simplified solution and the JPL DE405 ephemeris give the following results:
RMS max
Heliocentric:
     position error 3.7 11.2 km
     velocity error 1.4  5.0 mm/s
Barycentric:
     position error 4.6 13.4 km
     velocity error 1.4  4.9 mm/s

The results deteriorate outside this time span.

(5)
The routine sla_EVP is faster but less accurate. The present routine targets the case where high accuracy is more important than CPU time, yet the extra complication of reading a pre-computed ephemeris is not justified.