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<h3 class="section">39.17 Examples</h3>
<p>The following program demonstrates the use of the physical constants in
a calculation. In this case, the goal is to calculate the range of
light-travel times from Earth to Mars.
<p>The required data is the average distance of each planet from the Sun in
astronomical units (the eccentricities and inclinations of the orbits
will be neglected for the purposes of this calculation). The average
radius of the orbit of Mars is 1.52 astronomical units, and for the
orbit of Earth it is 1 astronomical unit (by definition). These values
are combined with the MKSA values of the constants for the speed of
light and the length of an astronomical unit to produce a result for the
shortest and longest light-travel times in seconds. The figures are
converted into minutes before being displayed.
<pre class="example"><pre class="verbatim"> #include <stdio.h>
#include <gsl/gsl_const_mksa.h>
int
main (void)
{
double c = GSL_CONST_MKSA_SPEED_OF_LIGHT;
double au = GSL_CONST_MKSA_ASTRONOMICAL_UNIT;
double minutes = GSL_CONST_MKSA_MINUTE;
/* distance stored in meters */
double r_earth = 1.00 * au;
double r_mars = 1.52 * au;
double t_min, t_max;
t_min = (r_mars - r_earth) / c;
t_max = (r_mars + r_earth) / c;
printf ("light travel time from Earth to Mars:\n");
printf ("minimum = %.1f minutes\n", t_min / minutes);
printf ("maximum = %.1f minutes\n", t_max / minutes);
return 0;
}
</pre></pre>
<p class="noindent">Here is the output from the program,
<pre class="example"><pre class="verbatim"> light travel time from Earth to Mars:
minimum = 4.3 minutes
maximum = 21.0 minutes
</pre></pre>
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