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<H2><A NAME="SECTION00043000000000000000">SLA_AIRMAS - Air Mass</A>
<A NAME="xref_SLA_AIRMAS"> </A><A NAME="SLA_AIRMAS"> </A>
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<DL>
<DT><STRONG>ACTION:</STRONG>
<DD>Air mass at given zenith distance (double precision).
<DT><STRONG>CALL:</STRONG>
<DD><TT>D = sla_AIRMAS (ZD)</TT>
<P> </DL>
<P> <DL>
<DT><STRONG>GIVEN:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>ZD</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>observed zenith distance (radians)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>RETURNED:</STRONG>
<DD>
<BR>
<TABLE CELLPADDING=3>
<TR VALIGN="TOP"><TD ALIGN="LEFT"><EM>sla_AIRMAS</EM></TD>
<TH ALIGN="LEFT"><B>D</B></TH>
<TD ALIGN="LEFT" NOWRAP>air mass (1 at zenith)</TD>
</TR>
</TABLE></DL>
<P> <DL>
<DT><STRONG>NOTES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>The <I>observed</I> zenith distance referred to above means
``as affected by refraction''.
<DT>2.
<DD>The routine uses Hardie's (1962) polynomial fit to Bemporad's
data for the relative air mass, <I>X</I>, in units of thickness at the
zenith as tabulated by Schoenberg (1929). This is adequate for all
normal needs as it is accurate to better than
0.1% up to <I>X</I> = 6.8 and better than 1% up to <I>X</I> = 10.
Bemporad's tabulated values are unlikely to be trustworthy
to such accuracy
because of variations in density, pressure and other
conditions in the atmosphere from those assumed in his work.
<DT>3.
<DD>The sign of the ZD is ignored.
<DT>4.
<DD>At zenith distances greater than about <IMG WIDTH="56" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img8.gif"
ALT="$\zeta = 87^{\circ}$"> the
air mass is held constant to avoid arithmetic overflows.
</DL></DL>
<P> <DL>
<DT><STRONG>REFERENCES:</STRONG>
<DD><DL COMPACT>
<DT>1.
<DD>Hardie, R.H., 1962, in <I>Astronomical Techniques</I>
ed. W.A. Hiltner, University of Chicago Press, p180.
<DT>2.
<DD>Schoenberg, E., 1929, Hdb. d. Ap.,
Berlin, Julius Springer, 2, 268.
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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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