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<H2><A NAME="SECTION00046000000000000000">SLA_AMPQK - Quick Apparent to Mean</A>
<A NAME="xref_SLA_AMPQK"> </A><A NAME="SLA_AMPQK"> </A>
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Convert star <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> from geocentric apparent to mean place
(post IAU 1976). Use of this routine is appropriate when
efficiency is important and where many star positions are
all to be transformed for one epoch and equinox. The
star-independent parameters can be obtained by calling
the sla_MAPPA routine.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_AMPQK (RA, DA, AMPRMS, RM, DM)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RA,DA</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>apparent <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>AMPRMS</EM></TD>
<TD ALIGN="LEFT"><B>D(21)</B></TD>
<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>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RM,DM</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>mean <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The accuracy is limited by the routine sla_EVP, called
by sla_MAPPA, which computes the Earth position and
velocity using the methods of Stumpff. The maximum
error is about 0.3 milliarcsecond.
<DT>2.
<DD>Iterative techniques are used for the aberration and
light deflection corrections so that the routines
sla_AMP (or sla_AMPQK) and sla_MAP (or sla_MAPQK) are
accurate inverses; even at the edge of the Sun's disc
the discrepancy is only about 1 nanoarcsecond.
</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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