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<H2><A NAME="SECTION000489000000000000000">SLA_EVP - Earth Position & Velocity</A>
<A NAME="xref_SLA_EVP"> </A><A NAME="SLA_EVP"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Barycentric and heliocentric velocity and position of the Earth.
<DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_EVP (DATE, DEQX, DVB, DPB, DVH, DPH)</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>TDB (formerly ET) as a Modified Julian Date
(JD-2400000.5)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DEQX</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>Julian Epoch (<I>e.g.</I> 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.</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DVB</EM></TD>
<TH ALIGN="LEFT"><B>D(3)</B></TH>
<TD ALIGN="LEFT" NOWRAP>barycentric <IMG WIDTH="58" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img111.gif"
ALT="$[\,\dot{x},\dot{y},\dot{z}\,]$">, AU s<SUP>-1</SUP></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DPB</EM></TD>
<TD ALIGN="LEFT"><B>D(3)</B></TD>
<TD ALIGN="LEFT" NOWRAP>barycentric <IMG WIDTH="58" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img50.gif"
ALT="$[\,x,y,z\,]$">, AU</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DVH</EM></TD>
<TD ALIGN="LEFT"><B>D(3)</B></TD>
<TD ALIGN="LEFT" NOWRAP>heliocentric <IMG WIDTH="58" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img111.gif"
ALT="$[\,\dot{x},\dot{y},\dot{z}\,]$">, AU s<SUP>-1</SUP></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DPH</EM></TD>
<TD ALIGN="LEFT"><B>D(3)</B></TD>
<TD ALIGN="LEFT" NOWRAP>heliocentric <IMG WIDTH="58" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img50.gif"
ALT="$[\,x,y,z\,]$">, AU</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>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:
<UL>
<LI> velocity (barycentric or heliocentric): 420 mm s<SUP>-1</SUP>
<LI> position (barycentric): 6900 km
<LI> position (heliocentric): 1600 km
</UL>
<DT>2.
<DD>The routine is an adaption of the BARVEL and BARCOR
subroutines of P.Stumpff, which are described in
<I>Astr.Astrophys.Suppl.Ser.</I> <B>41</B>, 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 <B>not</B> 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
<IMG WIDTH="39" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img112.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.001$"> from the value given in the
<I>Explanatory Supplement</I>.
</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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