1 /**************************************************************************
3 * Written by Durk Talsma. Originally started October 1997, for distribution
4 * with the FlightGear project. Version 2 was written in August and
5 * September 1998. This code is based upon algorithms and data kindly
6 * provided by Mr. Paul Schlyter. (pausch@saaf.se).
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License as
10 * published by the Free Software Foundation; either version 2 of the
11 * License, or (at your option) any later version.
13 * This program is distributed in the hope that it will be useful, but
14 * WITHOUT ANY WARRANTY; without even the implied warranty of
15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 * General Public License for more details.
18 * You should have received a copy of the GNU General Public License
19 * along with this program; if not, write to the Free Software
20 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
23 **************************************************************************/
25 #include "celestialBody.hxx"
27 #include <Debug/logstream.hxx>
29 #ifdef FG_MATH_EXCEPTION_CLASH
30 # define exception c_exception
34 /**************************************************************************
35 * void CelestialBody::updatePosition(fgTIME *t, Star *ourSun)
37 * Basically, this member function provides a general interface for
38 * calculating the right ascension and declinaion. This function is
39 * used for calculating the planetary positions. For the planets, an
40 * overloaded member function is provided to additionally calculate the
42 * The sun and moon have their own overloaded updatePosition member, as their
43 * position is calculated an a slightly different manner.
46 * fgTIME t: provides the current time.
47 * Star *ourSun: the sun's position is needed to convert heliocentric
48 * coordinates into geocentric coordinates.
52 *************************************************************************/
53 void CelestialBody::updatePosition(FGTime *t, Star *ourSun)
55 double eccAnom, v, ecl, actTime,
56 xv, yv, xh, yh, zh, xg, yg, zg, xe, ye, ze;
59 actTime = fgCalcActTime(t);
61 // calcualate the angle bewteen ecliptic and equatorial coordinate system
62 ecl = DEG_TO_RAD * (23.4393 - 3.563E-7 *actTime);
64 eccAnom = fgCalcEccAnom(M, e); //calculate the eccentric anomaly
65 xv = a * (cos(eccAnom) - e);
66 yv = a * (sqrt (1.0 - e*e) * sin(eccAnom));
67 v = atan2(yv, xv); // the planet's true anomaly
68 r = sqrt (xv*xv + yv*yv); // the planet's distance
70 // calculate the planet's position in 3D space
71 xh = r * (cos(N) * cos(v+w) - sin(N) * sin(v+w) * cos(i));
72 yh = r * (sin(N) * cos(v+w) + cos(N) * sin(v+w) * cos(i));
73 zh = r * (sin(v+w) * sin(i));
75 // calculate the ecliptic longitude and latitude
76 xg = xh + ourSun->getxs();
77 yg = yh + ourSun->getys();
80 lonEcl = atan2(yh, xh);
81 latEcl = atan2(zh, sqrt(xh*xh+yh*yh));
84 ye = yg * cos(ecl) - zg * sin(ecl);
85 ze = yg * sin(ecl) + zg * cos(ecl);
86 rightAscension = atan2(ye, xe);
87 declination = atan2(ze, sqrt(xe*xe + ye*ye));
88 FG_LOG(FG_GENERAL, FG_INFO, "Planet found at : "
89 << rightAscension << " (ra), " << declination << " (dec)" );
91 //calculate some variables specific to calculating the magnitude
93 R = sqrt (xg*xg + yg*yg + zg*zg);
94 s = ourSun->getDistance();
96 // It is possible from these calculations for the argument to acos
97 // to exceed the valid range for acos(). So we do a little extra
100 double tmp = (r*r + R*R - s*s) / (2*r*R);
103 } else if ( tmp < -1.0) {
107 FV = RAD_TO_DEG * acos( tmp );
110 /****************************************************************************
111 * double CelestialBody::fgCalcEccAnom(double M, double e)
112 * this private member calculates the eccentric anomaly of a celestial body,
113 * given its mean anomaly and eccentricity.
115 * -Mean anomaly: the approximate angle between the perihelion and the current
116 * position. this angle increases uniformly with time.
118 * True anomaly: the actual angle between perihelion and current position.
120 * Eccentric anomaly: this is an auxilary angle, used in calculating the true
121 * anomaly from the mean anomaly.
123 * -eccentricity. Indicates the amount in which the orbit deviates from a
124 * circle (0 = circle, 0-1, is ellipse, 1 = parabola, > 1 = hyperbola).
126 * This function is also known as solveKeplersEquation()
129 * M: the mean anomaly
130 * e: the eccentricity
133 * the eccentric anomaly
135 ****************************************************************************/
136 double CelestialBody::fgCalcEccAnom(double M, double e)
139 eccAnom, E0, E1, diff;
141 eccAnom = M + e * sin(M) * (1.0 + e * cos (M));
142 // iterate to achieve a greater precision for larger eccentricities
148 E1 = E0 - (E0 - e * sin(E0) - M) / (1 - e *cos(E0));
149 diff = fabs(E0 - E1);
152 while (diff > (DEG_TO_RAD * 0.001));