# Orbital Elements (C)¶

This is a simple overview of the helper functions in REBOUND for using orbital elements. For an in-depth discussion and examples, see ipython_examples/OrbitalElements.ipynb. The C function calls are more explicit (see below), but the numerical issues and conventions are the same in Python and C.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include "rebound.h"
#include "tools.h"

int main(int argc, char* argv[]){
struct reb_simulation* r = reb_create_simulation();

struct reb_particle p;
p.m      = 1.;

// Adding a particle with orbital elements requires the following 7 things
// There's more flexibility in python for passing different orbital elements
// for edge cases.  The user has to calculate these manually in C and pass
// the elements below

struct reb_particle primary = r->particles[0];
double m = 0.;
double a = 0.1;
double e = 0.2;
double inc = 0.3;
double Omega = 0.4;
double omega = 0.5;
double f = 0.6;

struct reb_particle p2 = reb_tools_orbit_to_particle(r->G, primary, m, a, e, inc, Omega, omega, f);

struct reb_orbit o= reb_tools_particle_to_orbit(r->G, r->particles[1], r->particles[0]);

printf("a = %.16e\n", o.a);
printf("e = %.16e\n", o.e);
printf("inc = %.16e\n", o.inc);
printf("Omega = %.16e\n", o.Omega);
printf("omega = %.16e\n", o.omega);
printf("f = %.16e\n", o.f);

// There are also versions of the two functions above that let you pass an error integer pointer
// to diagnose / catch errors.  You can find error codes in the documentation for the functions

int err = 0;
e = 1.001;

p2 = reb_tools_orbit_to_particle_err(r->G, primary, m, a, e, inc, Omega, omega, f, &err);
if(err == 3){            // error code for bound orbit with e > 1
e = 1.-1.e-15;        // set to just less than 1
p2 = reb_tools_orbit_to_particle_err(r->G, primary, m, a, e, inc, Omega, omega, f, &err);
}