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<H2><A NAME="SECTION0004183000000000000000"> </A><A NAME="xref_SLA_ZD"> </A><A NAME="SLA_ZD"> </A>
<BR>
SLA_ZD - <IMG WIDTH="27" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img65.gif"
ALT="$h,\delta$"> to Zenith Distance
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Hour angle and declination to zenith distance
(double precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>D = sla_ZD (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</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</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</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_ZD</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>zenith distance (radians, <IMG WIDTH="35" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img234.gif"
ALT="$0\!-\!\pi$">)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The latitude must be geodetic. In critical applications,
corrections for polar motion should be applied (see sla_POLMO).
<DT>2.
<DD>In some applications it will be important to specify the
correct type of hour angle and declination in order to
produce the required type
of zenith distance. In particular, it may be
important to distinguish between the zenith distance
as affected by refraction, which would require the
<I>observed</I> <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$">, and the zenith distance <I>in vacuo</I>,
which would require the <I>topocentric</I> <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$">. If
the effects of diurnal aberration can be neglected, the
<I>apparent</I> <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$"> may be used instead of the
<I>topocentric</I> <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$">. <DT>3.
<DD>No range checking of arguments is done.
<DT>4.
<DD>In applications which involve many zenith distance calculations,
rather than calling the present routine it will be more
efficient to use inline code, having previously computed fixed
terms such as sine and cosine of latitude, and perhaps sine and
cosine of declination.
</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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