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<H2><A NAME="SECTION0004117000000000000000">SLA_MOON - Approx Moon Pos/Vel</A>
<A NAME="xref_SLA_MOON"> </A><A NAME="SLA_MOON"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Approximate geocentric position and velocity of the Moon
(single precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_MOON (IY, ID, FD, PV)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>IY</EM></TD>
<TH ALIGN="LEFT"><B>I</B></TH>
<TD ALIGN="LEFT" NOWRAP>year</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>ID</EM></TD>
<TD ALIGN="LEFT"><B>I</B></TD>
<TD ALIGN="LEFT" NOWRAP>day in year (1 = Jan 1st)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>FD</EM></TD>
<TD ALIGN="LEFT"><B>R </B></TD>
<TD ALIGN="LEFT" NOWRAP>fraction of day</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>R(6)</B></TH>
<TD ALIGN="LEFT" NOWRAP>Moon <IMG WIDTH="106" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img51.gif"
ALT="$[\,x,y,z,\dot{x},\dot{y},\dot{z}\,]$">, mean equator and equinox of
date (AU, AU s<SUP>-1</SUP>)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The date and time is TDB (loosely ET) in a Julian calendar
which has been aligned to the ordinary Gregorian
calendar for the interval 1900 March 1 to 2100 February 28.
The year and day can be obtained by calling sla_CALYD or
sla_CLYD.
<DT>2.
<DD>The position is accurate to better than 0.5 arcminute
in direction and 1000 km in distance. The velocity
is accurate to better than
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img83.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.5$"> per hour in direction
and 4 metres per socond in distance. (RMS figures with respect
to JPL DE200 for the interval 1960-2025 are <IMG WIDTH="25" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img126.gif"
ALT="$14\hspace{-0.05em}^{'\hspace{-0.1em}'}$"> and
<P> <IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img76.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$"> per hour in longitude, <IMG WIDTH="17" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img127.gif"
ALT="$9\hspace{-0.05em}^{'\hspace{-0.1em}'}$"> and
<IMG WIDTH="23" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
SRC="img76.gif"
ALT="$0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$"><P>
per hour in latitude, 350 km and 2 metres per second in distance.)
Note that the distance accuracy is comparatively poor because this
routine is principally intended for computing topocentric direction.
<DT>3.
<DD>This routine is only a partial implementation of the original
Meeus algorithm (reference below), which offers 4 times the
accuracy in direction and 20 times the accuracy in distance
when fully implemented (as it is in sla_DMOON).
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCE:</STRONG>
<DD>Meeus, <I>l'Astronomie</I>, June 1984, p348.
</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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