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<H2><A NAME="SECTION0004114000000000000000">SLA_MAPPA - Mean to Apparent Parameters</A>
<A NAME="xref_SLA_MAPPA"> </A><A NAME="SLA_MAPPA"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Compute star-independent parameters in preparation for
conversions between mean place and geocentric apparent place.
The parameters produced by this routine are required in the
parallax, light deflection, aberration, and precession/nutation
parts of the mean/apparent transformations.
The reference frames and timescales used are post IAU 1976.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_MAPPA (EQ, DATE, AMPRMS)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EQ</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>epoch of mean equinox to be used (Julian)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DATE</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>TDB (JD-2400000.5)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>AMPRMS</EM></TD>
<TH ALIGN="LEFT"><B>D(21)</B></TH>
<TD ALIGN="LEFT" NOWRAP>star-independent mean-to-apparent parameters:</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(1)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>time interval for proper motion (Julian years)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(2-4)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>barycentric position of the Earth (AU)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(5-7)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>heliocentric direction of the Earth (unit vector)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(8)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>(gravitational radius of
Sun)<IMG WIDTH="31" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img16.gif"
ALT="$\times 2 / $">(Sun-Earth distance)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(9-11)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP><B>v</B>: barycentric Earth velocity in units of c</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(12)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="72" HEIGHT="45" ALIGN="MIDDLE" BORDER="0"
SRC="img17.gif"
ALT="$\sqrt{1-\left\vert\mbox{\bf v}\right\vert^2}$"></TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="CENTER" NOWRAP COLSPAN=1>(13-21)</TD>
<TD></TD>
<TD ALIGN="LEFT" NOWRAP>precession/nutation <IMG WIDTH="39" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img18.gif"
ALT="$3\times3$"> matrix</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>For DATE, the distinction between the required TDB and TT
is always negligible. Moreover, for all but the most
critical applications UTC is adequate.
<DT>2.
<DD>The accuracy of the routines using the parameters AMPRMS is
limited by the routine sla_EVP, used here to compute the
Earth position and velocity by the methods of Stumpff.
The maximum error in the resulting aberration corrections is
about 0.3 milliarcsecond.
<DT>3.
<DD>The vectors AMPRMS(2-4) and AMPRMS(5-7) are referred to
the mean equinox and equator of epoch EQ.
<DT>4.
<DD>The parameters produced by this routine are used by
sla_MAPQK and sla_MAPQKZ.
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>1984 <I>Astronomical Almanac</I>, pp B39-B41.
<DT>2.
<DD>Lederle & Schwan, 1984. <I>Astr.Astrophys.</I> <B>134</B>, 1-6.
</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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