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libnova-0.15.0/src/dynamical_time.c  view on Meta::CPAN

    16.0, 14.0, 13.0, 12.0, 11.0,
    10.0, 9.0, 9.0, 9.0, 9.0,
    9.0, 9.0, 9.0, 9.0, 10.0,
    10.0, 10.0, 10.0, 10.0, 11.0,
    11.0, 11.0, 11.0, 11.0, 11.0,
    11.0, 12.0, 12.0, 12.0, 12.0,
    12.0, 12.0, 13.0, 13.0, 13.0,
    13.0, 14.0, 14.0, 14.0, 15.0,
    15.0, 15.0, 15.0, 16.0, 16.0,
    16.0, 16.0, 16.0, 17.0, 17.0,
    17.0, 17.0, 17.0, 17.0, 17.0,
    17.0, 16.0, 16.0, 15.0, 14.0,
    13.7, 13.1, 12.7, 12.5, 12.5,
    12.5, 12.5, 12.5, 12.5, 12.3,
    12.0, 11.4, 10.6, 9.6, 8.6,
    7.5, 6.6, 6.0, 5.7, 5.6,
    5.7, 5.9, 6.2, 6.5, 6.8,
    7.1, 7.3, 7.5, 7.7, 7.8,
    7.9, 7.5, 6.4, 5.4, 2.9,
    1.6, -1.0, -2.7, -3.6, -4.7,
    -5.4, -5.2, -5.5, -5.6, -5.8,
    -5.9, -6.2, -6.4, -6.1, -4.7,
    -2.7, 0.0, 2.6, 5.4, 7.7,
    10.5, 13.4, 16.0, 18.2, 20.2,
    21.2, 22.4, 23.5, 23.9, 24.3,
    24.0, 23.9, 23.9, 23.7, 24.0,
    24.3, 25.3, 26.2, 27.3, 28.2,
    29.1, 30.0, 30.7, 31.4, 32.2,
    33.1, 34.0, 35.0, 36.5, 38.3,
    40.2, 42.2, 44.5, 46.5, 48.5,
    50.5, 52.2, 53.8, 54.9, 55.8,
    56.9, 58.3
    };
 					

/* Stephenson and Houlden  for years prior to 948 A.D.*/
static double get_dynamical_diff_sh1 (double JD)
{
    double TD,E;
    
    /* number of centuries from 948 */
    E = (JD - 2067314.5) / 36525.0;
    
    TD = 1830.0 - 405.0 * E + 46.5 * E * E;
    return (TD);
}

/* Stephenson and Houlden for years between 948 A.D. and 1600 A.D.*/
static double get_dynamical_diff_sh2 (double JD)
{
    double TD,t;
    
    /* number of centuries from 1850 */
    t = (JD - 2396758.5) / 36525.0;
    
    TD = 22.5 * t * t;
    return TD;
}

/* Table 9.a pg 72 for years 1600..1992.*/
/* uses interpolation formula 3.3 on pg 25 */
static double get_dynamical_diff_table (double JD)
{
    double TD = 0;
    double a,b,c,n;
    int i;
    
    /* get no days since 1620 and divide by 2 years */
    i = (int)((JD - 2312752.5) / 730.5);
    
    /* get the base interpolation factor in the table */
    if (i > (TERMS - 2))
        i = TERMS - 2;
	
	/* calc a,b,c,n */
	a = delta_t[i+1] - delta_t[i];
	b = delta_t[i+2] - delta_t[i+1];
	c = a - b;
	n = ((JD - (2312752.5 + (730.5 * i))) / 730.5);
	
	TD = delta_t[i+1] + n / 2 * (a + b + n * c);

    return TD;
}

/* get the dynamical time diff in the near past / future 1992 .. 2010 */
/* uses interpolation formula 3.3 on pg 25 */
static double get_dynamical_diff_near (double JD)
{
    double TD = 0;
    /* TD for 1990, 2000, 2010 */
    double delta_T[3] = {56.86, 63.83, 70.0};
    double a,b,c,n;
         
    /* calculate TD by interpolating value */
    a = delta_T[1] - delta_T[0];
    b = delta_T[2] - delta_T[1];
    c = b - a;
    
    /* get number of days since 2000 and divide by 10 years */
	n = (JD - 2451544.5) / 3652.5; 
	TD = delta_T[1] + (n / 2) * (a + b + n * c);
	     
    return TD;
} 

/* uses equation 9.1 pg 73 to calc JDE for othe JD values */          
static double get_dynamical_diff_other (double JD)
{     
    double TD;
    double a;
    
    a = (JD - 2382148);
    a *= a;

    TD = -15 + a / 41048480;
       
    return (TD);
}  

/*! \fn double ln_get_dynamical_time_diff (double JD)
* \param JD Julian Day
* \return TD
*
* Calculates the dynamical time (TD) difference in seconds (delta T) from 
* universal time.
*/
/* Equation 9.1 on pg 73.
*/
double ln_get_dynamical_time_diff (double JD)
{
    double TD;

    /* check when JD is, and use corresponding formula */
    /* check for date < 948 A.D. */
    if ( JD < 2067314.5 )
        /* Stephenson and Houlden */
	    TD = get_dynamical_diff_sh1 (JD);
    else if ( JD >= 2067314.5 && JD < 2305447.5 )
	    /* check for date 948..1600 A.D. Stephenson and Houlden */
    	TD = get_dynamical_diff_sh2 (JD);
	else if ( JD >= 2312752.5 && JD < 2448622.5 )
		/* check for value in table 1620..1992  interpolation of table */
		TD = get_dynamical_diff_table (JD);
	else if ( JD >= 2448622.5 && JD <= 2455197.5 )
		/* check for near future 1992..2010 interpolation */
		TD = get_dynamical_diff_near (JD);       
	else
	    /* other time period outside */
	    TD = get_dynamical_diff_other (JD);   	    
		    
	return TD;
}
      
     
/*! \fn double ln_get_jde (double JD)
* \param JD Julian Day
* \return Julian Ephemeris day 
*     
* Calculates the Julian Ephemeris Day (JDE) from the given julian day
*/     
    
double ln_get_jde (double JD)
{
    double JDE;
    double secs_in_day = 24 * 60 * 60;
    
    JDE = JD +  ln_get_dynamical_time_diff (JD) / secs_in_day;
    
    return JDE;
}