astrolibR (version 0.1)

baryvel: Calculates heliocentric and barycentric velocity components of Earth

Description

Calculates heliocentric and barycentric velocity components of Earth

Usage

baryvel(dje,deq)

Arguments

dje

Julian ephemeris date (scalar)

deq

epoch of mean equinox of dvelh and dvelb (scalar). If deq=0, then deq is assumed to be equal to dje.

Value

dvelh

heliocentric velocity components (3 element vector) in km/s

dvelb

barycentric velocity components (3 element vector) in km/s

Details

The 3-vectors DVELH and DVELB are given in a right-handed coordinate system with the +X axis toward the Vernal Equinox, and +Z axis toward the celestial pole.

The projected velocity towards the celestial object can be computed from: v = dvelb[1]*cos(dec)*cos(ra) + dvelb[2]*cos(dec)*sin(ra) + dvelb[3]*sin(dec)

The algorithm here is taken from FORTRAN program of Stumpff (1980). Stumpf claimed an accuracy of 42 cm/s for the velocity. A comparison with the JPL FORTRAN planetary ephemeris program PLEPH found agreement to within about 65 cm/s between 1986 and 1994. The option in IDL astrolib's baryvel.pro to use the full JPL ephemeris is not implemented in R.

References

Two Self-Consistent FORTRAN Subroutines for the Computation of the Earth's Motion, P. Stumpff, Astronomy & Astrophysics Supplement, 41, 1, 1980.

Examples

baryvel(2456469.5, 0)