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<H2><A NAME="SECTION0004140000000000000000">SLA_PRECES - Precession</A>
<A NAME="xref_SLA_PRECES"> </A><A NAME="SLA_PRECES"> </A>
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<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Precession - either the old ``FK4'' (Bessel-Newcomb, pre IAU 1976)
or new ``FK5'' (Fricke, post IAU 1976) as required.
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_PRECES (SYSTEM, EP0, EP1, RA, DC)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>SYSTEM</EM></TD>
<TH ALIGN="LEFT"><B>C</B></TH>
<TD ALIGN="LEFT" NOWRAP>precession to be applied: `FK4' or `FK5'</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EP0,EP1</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>starting and ending epoch</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RA,DC</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">, mean equator & equinox of epoch EP0</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>RA,DC</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP><IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">, mean equator & equinox of epoch EP1</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Lowercase characters in SYSTEM are acceptable.
<DT>2.
<DD>The epochs are Besselian if SYSTEM=`FK4' and Julian if `FK5'.
For example, to precess coordinates in the old system from
equinox 1900.0 to 1950.0 the call would be:
<BLOCKQUOTE><TT>CALL sla_PRECES ('FK4', 1900D0, 1950D0, RA, DC)</TT>
</BLOCKQUOTE>
<DT>3.
<DD>This routine will <B>NOT</B> correctly convert between the old and
the new systems - for example conversion from B1950 to J2000.
For these purposes see sla_FK425, sla_FK524, sla_FK45Z and
sla_FK54Z.
<DT>4.
<DD>If an invalid SYSTEM is supplied, values of -99D0,-99D0 are
returned for both RA and DC.
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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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