// Some constants useful in solar system
#define mmoon 7.342e25           // Moon mass in g
#define mearth 5.97219e27        // Earth mass in g 
#define msun 1.989e33            // Sun mass in g 
#define au 1.495978707e13        // au in cm 
#define rearth 6371e5            // Earth dameter in cm 
#define dmoon 384400e5           // Distance moon in cm 
#define sec_per_year 3.1556926e7 // seconds per year 
#define parsec 3.085678e18       // cm per parsec
#define kparsec 3.085678e21      // cm per kilo parsec
#define mparsec 3.085678e24      // cm per mega parsec

// unit system of the simulation (solar system)
//#define m_unit msun
//#define l_unit au
//#define t_unit sec_per_year        // Years
//#define v_unit (l_unit / t_unit)   // resulting velocity unit
      
// unit system of the simulation (galaxies)
#define m_unit 1e10*msun
#define l_unit kparsec
#define v_unit 1e5                 // km/s 
#define t_unit (l_unit / v_unit)   // resulting time unit
      
// Gravitational constant and c in internal units 
#define G 6.672e-8 * m_unit * t_unit * t_unit / l_unit / l_unit / l_unit
#define c 2.99792458e+10 * t_unit / l_unit