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#include <stdio.h>
#include <math.h>
#include "astro.h"
/* correct the true altitude, ta, for refraction to the apparent altitude, aa,
* each in radians, given the local atmospheric pressure, pr, in mbars, and
* the temperature, tr, in degrees C.
*/
refract (pr, tr, ta, aa)
double pr, tr;
double ta;
double *aa;
{
double r; /* refraction correction*/
if (ta >= degrad(15.)) {
/* model for altitudes at least 15 degrees above horizon */
r = 7.888888e-5*pr/((273+tr)*tan(ta));
} else if (ta > degrad(-5.)) {
/* hairier model for altitudes at least -5 and below 15 degrees */
double a, b, tadeg = raddeg(ta);
a = ((2e-5*tadeg+1.96e-2)*tadeg+1.594e-1)*pr;
b = (273+tr)*((8.45e-2*tadeg+5.05e-1)*tadeg+1);
r = degrad(a/b);
} else {
/* do nothing if more than 5 degrees below horizon.
*/
r = 0;
}
*aa = ta + r;
}
/* correct the apparent altitude, aa, for refraction to the true altitude, ta,
* each in radians, given the local atmospheric pressure, pr, in mbars, and
* the temperature, tr, in degrees C.
*/
unrefract (pr, tr, aa, ta)
double pr, tr;
double aa;
double *ta;
{
double err;
double appar;
double true;
/* iterative solution: search for the true that refracts to the
* given apparent.
* since refract() is discontinuous at -5 degrees, there is a range
* of apparent altitudes between about -4.5 and -5 degrees that are
* not invertable (the graph of ap vs. true has a vertical step at
* true = -5). thus, the iteration just oscillates if it gets into
* this region. if this happens the iteration is forced to abort.
* of course, this makes unrefract() discontinuous too.
*/
true = aa;
do {
refract (pr, tr, true, &appar);
err = appar - aa;
true -= err;
} while (fabs(err) >= 1e-6 && true > degrad(-5));
*ta = true;
}
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.