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<H2><A NAME="SECTION00045000000000000000">SLA_AMP - Apparent to Mean</A>
<A NAME="xref_SLA_AMP"> </A><A NAME="SLA_AMP"> </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).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_AMP (RA, DA, DATE, EQ, 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>DATE</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>TDB for apparent place (JD-2400000.5)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EQ</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>equinox: Julian epoch of mean place</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 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 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>3.
<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.
<DT>4.
<DD>Where multiple apparent places are to be converted to
mean places, for a fixed date and equinox, it is more
efficient to use the sla_MAPPA routine to compute the
required parameters once, followed by one call to
sla_AMPQK per star.
</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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