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.help planet Jun99 "Slalib Package"
.nf
SUBROUTINE slPLNT (DATE, NP, PV, JSTAT)
- - - - - - -
P L N T
- - - - - - -
Approximate heliocentric position and velocity of a specified
major planet.
Given:
DATE d Modified Julian Date (JD - 2400000.5)
NP i planet (1=Mercury, 2=Venus, 3=EMB ... 9=Pluto)
Returned:
PV d(6) heliocentric x,y,z,xdot,ydot,zdot, J2000
equatorial triad (AU,AU/s)
JSTAT i status: +1 = warning: date out of range
0 = OK
-1 = illegal NP (outside 1-9)
-2 = solution didn't converge
Called: slPLNE
Notes
1 The epoch, DATE, is in the TDB timescale and is a Modified
Julian Date (JD-2400000.5).
2 The reference frame is equatorial and is with respect to the
mean equinox and ecliptic of epoch J2000.
3 If an NP value outside the range 1-9 is supplied, an error
status (JSTAT = -1) is returned and the PV vector set to zeroes.
4 The algorithm for obtaining the mean elements of the planets
from Mercury to Neptune is due to J.L. Simon, P. Bretagnon,
J. Chapront, M. Chapront-Touze, G. Francou and J. Laskar
(Bureau des Longitudes, Paris). The (completely different)
algorithm for calculating the ecliptic coordinates of Pluto
is by Meeus.
5 Comparisons of the present routine with the JPL DE200 ephemeris
give the following RMS errors over the interval 1960-2025:
position (km) speed (metre/sec)
Mercury 334 0.437
Venus 1060 0.855
EMB 2010 0.815
Mars 7690 1.98
Jupiter 71700 7.70
Saturn 199000 19.4
Uranus 564000 16.4
Neptune 158000 14.4
Pluto 36400 0.137
From comparisons with DE102, Simon et al quote the following
longitude accuracies over the interval 1800-2200:
Mercury 4"
Venus 5"
EMB 6"
Mars 17"
Jupiter 71"
Saturn 81"
Uranus 86"
Neptune 11"
In the case of Pluto, Meeus quotes an accuracy of 0.6 arcsec
in longitude and 0.2 arcsec in latitude for the period
1885-2099.
For all except Pluto, over the period 1000-3000 the accuracy
is better than 1.5 times that over 1800-2200. Outside the
period 1000-3000 the accuracy declines. For Pluto the
accuracy declines rapidly outside the period 1885-2099.
Outside these ranges (1885-2099 for Pluto, 1000-3000 for
the rest) a "date out of range" warning status (JSTAT=+1)
is returned.
6 The algorithms for (i) Mercury through Neptune and (ii) Pluto
are completely independent. In the Mercury through Neptune
case, the present SLALIB implementation differs from the
original Simon et al Fortran code in the following respects.
* The date is supplied as a Modified Julian Date rather
than a Julian Date (MJD = JD - 2400000.5).
* The result is returned only in equatorial Cartesian form;
the ecliptic longitude, latitude and radius vector are not
returned.
* The velocity is in AU per second, not AU per day.
* Different error/warning status values are used.
* Kepler's equation is not solved inline.
* Polynomials in T are nested to minimize rounding errors.
* Explicit double-precision constants are used to avoid
mixed-mode expressions.
* There are other, cosmetic, changes to comply with
Starlink/SLALIB style guidelines.
None of the above changes affects the result significantly.
7 For NP=3 the result is for the Earth-Moon Barycentre. To
obtain the heliocentric position and velocity of the Earth,
either use the SLALIB routine slEVP or call slDMON and
subtract 0.012150581 times the geocentric Moon vector from
the EMB vector produced by the present routine. (The Moon
vector should be precessed to J2000 first, but this can
be omitted for modern epochs without introducing significant
inaccuracy.)
References: Simon et al., Astron. Astrophys. 282, 663 (1994).
Meeus, Astronomical Algorithms, Willmann-Bell (1991).
P.T.Wallace Starlink 27 May 1997
Copyright (C) 1997 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
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