static int FindEclipticCrossings(Observer observer, AstroTime time)
{
int i;
var hor = new Spherical[NUM_SAMPLES];
/*
* The ecliptic is a celestial circle that describes the mean plane of
* the Earth's orbit around the Sun. We use J2000 ecliptic coordinates,
* meaning the x-axis is defined to where the plane of the Earth's
* equator on January 1, 2000 at noon UTC intersects the ecliptic plane.
* The positive x-axis points toward the March equinox.
* Calculate a rotation matrix that converts J2000 ecliptic vectors
* to horizontal vectors for this observer and time.
*/
RotationMatrix rot = Astronomy.Rotation_ECL_HOR(time, observer);
/*
* Sample several points around the ecliptic.
* Remember the horizontal coordinates for each sample.
*/
for (i = 0; i < NUM_SAMPLES; ++i)
{
if (0 != HorizontalCoords(out hor[i], ECLIPLON(i), time, rot))
{
return(1); /* failure */
}
}
for (i = 0; i < NUM_SAMPLES; ++i)
{
double a1 = hor[i].lat;
double a2 = hor[(i + 1) % NUM_SAMPLES].lat;
double e1 = ECLIPLON(i);
double e2 = ECLIPLON(i + 1);
double ex;
Spherical hx;
int error;
if (a1 * a2 <= 0.0)
{
/* Looks like a horizon crossing. Is altitude going up with longitude or down? */
if (a2 > a1)
{
/* Search for the ecliptic longitude and azimuth where altitude ascends through zero. */
error = Search(out ex, out hx, time, rot, e1, e2);
}
else
{
/* Search for the ecliptic longitude and azimuth where altitude descends through zero. */
error = Search(out ex, out hx, time, rot, e2, e1);
}
if (error != 0)
{
return(error);
}
string direction;
if (hx.lon > 0.0 && hx.lon < 180.0)
{
direction = "ascends "; /* azimuth is more toward the east than the west */
}
else
{
direction = "descends"; /* azimuth is more toward the west than the east */
}
Console.WriteLine("Ecliptic longitude {0,9:0.0000} {1} through horizon az {2,9:0.0000}, alt {3,12:0.000e+00}", ex, direction, hx.lon, hx.lat);
}
}
return(0);
}