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<H2><A NAME="SECTION0004134000000000000000">SLA_PLANET - Planetary Ephemerides</A>
<A NAME="xref_SLA_PLANET"> </A><A NAME="SLA_PLANET"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Approximate heliocentric position and velocity of a planet.
<DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_PLANET (DATE, NP, PV, JSTAT)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DATE</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>Modified Julian Date (JD-2400000.5)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>NP</EM></TD>
<TD ALIGN="LEFT"><B>I</B></TD>
<TD ALIGN="LEFT" NOWRAP>planet:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 1=Mercury</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 2=Venus</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 3=Earth-Moon Barycentre</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 4=Mars</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 5=Jupiter</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 6=Saturn</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 7=Uranus</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 8=Neptune</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 9=Pluto</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>PV</EM></TD>
<TH ALIGN="LEFT"><B>D(6)</B></TH>
<TD ALIGN="LEFT" NOWRAP>heliocentric <IMG WIDTH="106" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img51.gif"
ALT="$[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$">, equatorial, J2000</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> (AU, AU/s)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>JSTAT</EM></TD>
<TD ALIGN="LEFT"><B>I</B></TD>
<TD ALIGN="LEFT" NOWRAP>status:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> +1 = warning: date outside of range</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> 0 = OK</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> -1 = illegal NP (outside 1-9)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM></EM></TD>
<TD ALIGN="LEFT"><B></B></TD>
<TD ALIGN="LEFT" NOWRAP> -2 = solution didn't converge</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The epoch, DATE, is in the TDB timescale and is in the form
of a Modified Julian Date (JD-2400000.5).
<DT>2.
<DD>The reference frame is equatorial and is with respect to
the mean equinox and ecliptic of epoch J2000.
<DT>3.
<DD>If a planet number, NP, outside the range 1-9 is supplied, an error
status is returned (JSTAT = -1) and the PV vector
is set to zeroes.
<DT>4.
<DD>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, France). The (completely different)
algorithm for calculating the ecliptic coordinates of
Pluto is by Meeus.
<DT>5.
<DD>Comparisons of the present routine with the JPL DE200 ephemeris
give the following RMS errors over the interval 1960-2025:
<PRE><TT>
<I>position (km)</I> <I>speed (metre/sec)</I>
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
</TT></PRE>
From comparisons with DE102, Simon <I>et al.</I> quote the following
longitude accuracies over the interval 1800-2200:
<PRE><TT>
Mercury <IMG WIDTH="17" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img130.gif"
ALT="$4\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Venus <IMG WIDTH="17" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img131.gif"
ALT="$5\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
EMB <IMG WIDTH="17" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img132.gif"
ALT="$6\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Mars <IMG WIDTH="25" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img133.gif"
ALT="$17\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Jupiter <IMG WIDTH="25" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img134.gif"
ALT="$71\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Saturn <IMG WIDTH="25" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img135.gif"
ALT="$81\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Uranus <IMG WIDTH="25" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img136.gif"
ALT="$86\hspace{-0.05em}^{'\hspace{-0.1em}'}$">
Neptune <IMG WIDTH="25" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img43.gif"
ALT="$11\hspace{-0.05em}^{'\hspace{-0.1em}'}$"></TT></PRE>
In the case of Pluto, Meeus quotes an accuracy of
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img137.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.6$"><P>
in longitude and
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img76.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$"> in latitude for the period
1885-2099.
<P>
For all except Pluto, over the period 1000-3000,
the accuracy is better than 1.5
times that over 1800-2200. Outside the interval 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 (<TT>JSTAT=+1</TT>) is returned.
<DT>6.
<DD>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 <I>et al.</I> Fortran code in the following respects:
<UL>
<LI> The date is supplied as a Modified Julian Date rather
a Julian Date (<IMG WIDTH="177" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img138.gif"
ALT="${\rm MJD} = ({\rm JD} - 2400000.5$">).
<LI> The result is returned only in equatorial
Cartesian form; the ecliptic
longitude, latitude and radius vector are not returned.
<LI> The velocity is in AU per second, not AU per day.
<LI> Different error/warning status values are used.
<LI> Kepler's Equation is not solved inline.
<LI> Polynomials in T are nested to minimize rounding errors.
<LI> Explicit double-precision constants are used to avoid
mixed-mode expressions.
<LI> There are other, cosmetic, changes to comply with
Starlink/SLALIB style guidelines.
</UL>
None of the above changes affects the result significantly.
<DT>7.
<DD>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 sla_EVP or call sla_DMOON 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.)
</DL>
<P> <DL>
<DT><STRONG>REFERENCES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Simon <I>et al.,</I>
Astron. Astrophys. <B>282</B>, 663 (1994).
<DT>2.
<DD>Meeus, J.,
<I>Astronomical Algorithms,</I> Willmann-Bell (1991).
</DL></DL></DL>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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