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<H2><A NAME="SECTION0004125000000000000000"> </A><A NAME="xref_SLA_PA"> </A><A NAME="SLA_PA"> </A>
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SLA_PA - <IMG WIDTH="27" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img65.gif"
ALT="$h,\delta$"> to Parallactic Angle
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Hour angle and declination to parallactic angle
(double precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>D = sla_PA (HA, DEC, PHI)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>HA</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>hour angle in radians (geocentric apparent)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>DEC</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>declination in radians (geocentric apparent)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>PHI</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>latitude in radians (geodetic)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_PA</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>parallactic angle (radians, in the range <IMG WIDTH="25" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img47.gif"
ALT="$\pm \pi$">)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The parallactic angle at a point in the sky is the position
angle of the vertical, <I>i.e.</I> the angle between the direction to
the pole and to the zenith. In precise applications care must
be taken only to use geocentric apparent <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$"> and to consider
separately the effects of atmospheric refraction and telescope
mount errors.
<DT>2.
<DD>At the pole a zero result is returned.
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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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