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<H2><A NAME="SECTION000413000000000000000">SLA_BEAR - Direction Between Points on a Sphere</A>
<A NAME="xref_SLA_BEAR"> </A><A NAME="SLA_BEAR"> </A>
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<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Returns the bearing (position angle) of one point on a
sphere seen from another (single precision).
<P> <DT><STRONG>CALL:</STRONG>
<DD><TT>R = sla_BEAR (A1, B1, A2, B2)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>A1,B1</EM></TD>
<TH ALIGN="LEFT"><B>R</B></TH>
<TD ALIGN="LEFT" NOWRAP>spherical coordinates of one point</TD>
</TR>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>A2,B2</EM></TD>
<TD ALIGN="LEFT"><B>R</B></TD>
<TD ALIGN="LEFT" NOWRAP>spherical coordinates of the other point</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_BEAR</EM></TD>
<TH ALIGN="LEFT"><B>R</B></TH>
<TD ALIGN="LEFT" NOWRAP>bearing from first point to second</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The spherical coordinates are <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">,<IMG WIDTH="38" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img46.gif"
ALT="$[\lambda,\phi]$"> <I>etc.</I>, in radians.
<DT>2.
<DD>The result is the bearing (position angle), in radians,
of point [A2,B2] as seen
from point [A1,B1]. It is in the range <IMG WIDTH="25" HEIGHT="25" ALIGN="MIDDLE" BORDER="0"
SRC="img47.gif"
ALT="$\pm \pi$">. The sense
is such that if [A2,B2]
is a small distance due east of [A1,B1] the result
is about <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img14.gif"
ALT="$+\pi/2$">. Zero is returned
if the two points are coincident.
<DT>3.
<DD>If either B-coordinate is outside the range <IMG WIDTH="41" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img15.gif"
ALT="$\pm\pi/2$">, the
result may correspond to ``the long way round''.
<DT>4.
<DD>The routine sla_PAV performs an equivalent function except
that the points are specified in the form of Cartesian unit
vectors.
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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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