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 * (Log is kept at end of this file)
24 **************************************************************************/
26 #include "celestialBody.hxx"
28 #include <Debug/logstream.hxx>
30 #ifdef FG_MATH_EXCEPTION_CLASH
31 # define exception c_exception
35 /**************************************************************************
36 * void CelestialBody::updatePosition(fgTIME *t, Star *ourSun)
38 * Basically, this member function provides a general interface for
39 * calculating the right ascension and declinaion. This function is
40 * used for calculating the planetary positions. For the planets, an
41 * overloaded member function is provided to additionally calculate the
43 * The sun and moon have their own overloaded updatePosition member, as their
44 * position is calculated an a slightly different manner.
47 * fgTIME t: provides the current time.
48 * Star *ourSun: the sun's position is needed to convert heliocentric
49 * coordinates into geocentric coordinates.
53 *************************************************************************/
54 void CelestialBody::updatePosition(fgTIME *t, Star *ourSun)
56 double eccAnom, v, ecl, actTime,
57 xv, yv, xh, yh, zh, xg, yg, zg, xe, ye, ze;
60 actTime = fgCalcActTime(t);
62 // calcualate the angle bewteen ecliptic and equatorial coordinate system
63 ecl = DEG_TO_RAD * (23.4393 - 3.563E-7 *actTime);
65 eccAnom = fgCalcEccAnom(M, e); //calculate the eccentric anomaly
66 xv = a * (cos(eccAnom) - e);
67 yv = a * (sqrt (1.0 - e*e) * sin(eccAnom));
68 v = atan2(yv, xv); // the planet's true anomaly
69 r = sqrt (xv*xv + yv*yv); // the planet's distance
71 // calculate the planet's position in 3D space
72 xh = r * (cos(N) * cos(v+w) - sin(N) * sin(v+w) * cos(i));
73 yh = r * (sin(N) * cos(v+w) + cos(N) * sin(v+w) * cos(i));
74 zh = r * (sin(v+w) * sin(i));
76 // calculate the ecliptic longitude and latitude
77 xg = xh + ourSun->getxs();
78 yg = yh + ourSun->getys();
81 lonEcl = atan2(yh, xh);
82 latEcl = atan2(zh, sqrt(xh*xh+yh*yh));
85 ye = yg * cos(ecl) - zg * sin(ecl);
86 ze = yg * sin(ecl) + zg * cos(ecl);
87 rightAscension = atan2(ye, xe);
88 declination = atan2(ze, sqrt(xe*xe + ye*ye));
89 FG_LOG(FG_GENERAL, FG_INFO, "Planet found at : "
90 << rightAscension << " (ra), " << declination << " (dec)" );
92 //calculate some variables specific to calculating the magnitude
94 R = sqrt (xg*xg + yg*yg + zg*zg);
95 s = ourSun->getDistance();
97 // It is possible from these calculations for the argument to acos
98 // to exceed the valid range for acos(). So we do a little extra
101 double tmp = (r*r + R*R - s*s) / (2*r*R);
104 } else if ( tmp < -1.0) {
108 FV = RAD_TO_DEG * acos( tmp );
111 /****************************************************************************
112 * double CelestialBody::fgCalcEccAnom(double M, double e)
113 * this private member calculates the eccentric anomaly of a celestial body,
114 * given its mean anomaly and eccentricity.
116 * -Mean anomaly: the approximate angle between the perihelion and the current
117 * position. this angle increases uniformly with time.
119 * True anomaly: the actual angle between perihelion and current position.
121 * Eccentric anomaly: this is an auxilary angle, used in calculating the true
122 * anomaly from the mean anomaly.
124 * -eccentricity. Indicates the amount in which the orbit deviates from a
125 * circle (0 = circle, 0-1, is ellipse, 1 = parabola, > 1 = hyperbola).
127 * This function is also known as solveKeplersEquation()
130 * M: the mean anomaly
131 * e: the eccentricity
134 * the eccentric anomaly
136 ****************************************************************************/
137 double CelestialBody::fgCalcEccAnom(double M, double e)
140 eccAnom, E0, E1, diff;
142 eccAnom = M + e * sin(M) * (1.0 + e * cos (M));
143 // iterate to achieve a greater precision for larger eccentricities
149 E1 = E0 - (E0 - e * sin(E0) - M) / (1 - e *cos(E0));
150 diff = fabs(E0 - E1);
153 while (diff > (DEG_TO_RAD * 0.001));