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<H2><A NAME="SECTION000440000000000000000"> </A><A NAME="xref_SLA_DE2H"> </A><A NAME="SLA_DE2H"> </A>
<BR>
SLA_DE2H - <IMG WIDTH="27" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img65.gif"
ALT="$h,\delta$"> to Az,El
</H2>
<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Equatorial to horizon coordinates
(double precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>CALL sla_DE2H (HA, DEC, PHI, AZ, EL)</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 (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 (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 (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>AZ</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>azimuth (radians)</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>EL</EM></TD>
<TD ALIGN="LEFT"><B>D</B></TD>
<TD ALIGN="LEFT" NOWRAP>elevation (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Azimuth is returned in the range <IMG WIDTH="43" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img66.gif"
ALT="$0\!-\!2\pi$">; north is zero,
and east is <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img14.gif"
ALT="$+\pi/2$">. Elevation is returned in the range
<IMG WIDTH="25" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img47.gif"
ALT="$\pm \pi$">. <DT>2.
<DD>The latitude must be geodetic. In critical applications,
corrections for polar motion should be applied.
<DT>3.
<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 azimuth and elevation. In
particular, it may be important to distinguish between
elevation 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 elevation
<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 topocentric
<IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img29.gif"
ALT="$[\,h,\delta\,]$">. <DT>4.
<DD>No range checking of arguments is carried out.
<DT>5.
<DD>In applications which involve many such 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 (for tracking a star)
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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