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 /**************************************************************************
31 * void CelestialBody::updatePosition(fgTIME *t, Star *ourSun)
33 * Basically, this member function provides a general interface for
34 * calculating the right ascension and declinaion. This function is
35 * used for calculating the planetary positions. For the planets, an
36 * overloaded member function is provided to additionally calculate the
38 * The sun and moon have their own overloaded updatePosition member, as their
39 * position is calculated an a slightly different manner.
42 * fgTIME t: provides the current time.
43 * Star *ourSun: the sun's position is needed to convert heliocentric
44 * coordinates into geocentric coordinates.
48 *************************************************************************/
49 void CelestialBody::updatePosition(fgTIME *t, Star *ourSun)
51 double eccAnom, v, ecl, actTime,
52 xv, yv, xh, yh, zh, xg, yg, zg, xe, ye, ze;
55 actTime = fgCalcActTime(t);
57 // calcualate the angle bewteen ecliptic and equatorial coordinate system
58 ecl = DEG_TO_RAD * (23.4393 - 3.563E-7 *actTime);
60 eccAnom = fgCalcEccAnom(M, e); //calculate the eccentric anomaly
61 xv = a * (cos(eccAnom) - e);
62 yv = a * (sqrt (1.0 - e*e) * sin(eccAnom));
63 v = atan2(yv, xv); // the planet's true anomaly
64 r = sqrt (xv*xv + yv*yv); // the planet's distance
66 // calculate the planet's position in 3D space
67 xh = r * (cos(N) * cos(v+w) - sin(N) * sin(v+w) * cos(i));
68 yh = r * (sin(N) * cos(v+w) + cos(N) * sin(v+w) * cos(i));
69 zh = r * (sin(v+w) * sin(i));
71 // calculate the ecliptic longitude and latitude
72 xg = xh + ourSun->getxs();
73 yg = yh + ourSun->getys();
77 ye = yg * cos(ecl) - zg * sin(ecl);
78 ze = yg * sin(ecl) + zg * cos(ecl);
79 rightAscension = atan2(ye, xe);
80 declination = atan2(ze, sqrt(xe*xe + ye*ye));
81 FG_LOG(FG_GENERAL, FG_INFO, "Planet found at : "
82 << rightAscension << " (ra), " << declination << " (dec)" );
84 //calculate some variables specific to calculating the magnitude
86 R = sqrt (xg*xg + yg*yg + zg*zg);
87 s = ourSun->getDistance();
88 FV = RAD_TO_DEG * acos( (r*r + R*R - s*s) / (2*r*R));
91 /****************************************************************************
92 * double CelestialBody::fgCalcEccAnom(double M, double e)
93 * this private member calculates the eccentric anomaly of a celestial body,
94 * given its mean anomaly and eccentricity.
96 * -Mean anomaly: the approximate angle between the perihelion and the current
97 * position. this angle increases uniformly with time.
99 * True anomaly: the actual angle between perihelion and current position.
101 * Eccentric anomaly: this is an auxilary angle, used in calculating the true
102 * anomaly from the mean anomaly.
104 * -eccentricity. Indicates the amount in which the orbit deviates from a
105 * circle (0 = circle, 0-1, is ellipse, 1 = parabola, > 1 = hyperbola).
107 * This function is also known as solveKeplersEquation()
110 * M: the mean anomaly
111 * e: the eccentricity
114 * the eccentric anomaly
116 ****************************************************************************/
117 double CelestialBody::fgCalcEccAnom(double M, double e)
120 eccAnom, E0, E1, diff;
122 eccAnom = M + e * sin(M) * (1.0 + e * cos (M));
123 // iterate to achieve a greater precision for larger eccentricities
129 E1 = E0 - (E0 - e * sin(E0) - M) / (1 - e *cos(E0));
130 diff = fabs(E0 - E1);
133 while (diff > (DEG_TO_RAD * 0.001));