| 1 | /* functions to manipulate the modified-julian-date used throughout xephem. */
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| 2 |
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| 3 | #include <stdio.h>
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| 4 | #include <math.h>
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| 5 |
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| 6 | #include "astro.h"
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| 7 |
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| 8 | /* given a date in months, mn, days, dy, years, yr,
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| 9 | * return the modified Julian date (number of days elapsed since 1900 jan 0.5),
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| 10 | * *mjd.
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| 11 | */
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| 12 | void
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| 13 | cal_mjd (int mn, double dy, int yr, double *mjp)
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| 14 | {
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| 15 | static double last_mjd, last_dy;
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| 16 | static int last_mn, last_yr;
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| 17 | int b, d, m, y;
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| 18 | long c;
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| 19 |
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| 20 | if (mn == last_mn && yr == last_yr && dy == last_dy) {
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| 21 | *mjp = last_mjd;
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| 22 | return;
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| 23 | }
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| 24 |
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| 25 | m = mn;
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| 26 | y = (yr < 0) ? yr + 1 : yr;
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| 27 | if (mn < 3) {
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| 28 | m += 12;
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| 29 | y -= 1;
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| 30 | }
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| 31 |
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| 32 | if (yr < 1582 || (yr == 1582 && (mn < 10 || (mn == 10 && dy < 15))))
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| 33 | b = 0;
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| 34 | else {
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| 35 | int a;
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| 36 | a = y/100;
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| 37 | b = 2 - a + a/4;
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| 38 | }
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| 39 |
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| 40 | if (y < 0)
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| 41 | c = (long)((365.25*y) - 0.75) - 694025L;
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| 42 | else
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| 43 | c = (long)(365.25*y) - 694025L;
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| 44 |
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| 45 | d = (int)(30.6001*(m+1));
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| 46 |
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| 47 | *mjp = b + c + d + dy - 0.5;
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| 48 |
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| 49 | last_mn = mn;
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| 50 | last_dy = dy;
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| 51 | last_yr = yr;
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| 52 | last_mjd = *mjp;
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| 53 | }
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| 54 |
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| 55 | /* given the modified Julian date (number of days elapsed since 1900 jan 0.5,),
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| 56 | * mj, return the calendar date in months, *mn, days, *dy, and years, *yr.
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| 57 | */
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| 58 | void
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| 59 | mjd_cal (double mj, int *mn, double *dy, int *yr)
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| 60 | {
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| 61 | static double last_mj, last_dy;
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| 62 | static int last_mn, last_yr;
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| 63 | double d, f;
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| 64 | double i, a, b, ce, g;
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| 65 |
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| 66 | /* we get called with 0 quite a bit from unused epoch fields.
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| 67 | * 0 is noon the last day of 1899.
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| 68 | */
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| 69 | if (mj == 0.0) {
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| 70 | *mn = 12;
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| 71 | *dy = 31.5;
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| 72 | *yr = 1899;
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| 73 | return;
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| 74 | }
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| 75 |
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| 76 | if (mj == last_mj) {
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| 77 | *mn = last_mn;
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| 78 | *yr = last_yr;
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| 79 | *dy = last_dy;
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| 80 | return;
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| 81 | }
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| 82 |
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| 83 | d = mj + 0.5;
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| 84 | i = floor(d);
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| 85 | f = d-i;
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| 86 | if (f == 1) {
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| 87 | f = 0;
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| 88 | i += 1;
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| 89 | }
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| 90 |
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| 91 | if (i > -115860.0) {
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| 92 | a = floor((i/36524.25)+.99835726)+14;
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| 93 | i += 1 + a - floor(a/4.0);
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| 94 | }
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| 95 |
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| 96 | b = floor((i/365.25)+.802601);
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| 97 | ce = i - floor((365.25*b)+.750001)+416;
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| 98 | g = floor(ce/30.6001);
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| 99 | *mn = (int)(g - 1);
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| 100 | *dy = ce - floor(30.6001*g)+f;
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| 101 | *yr = (int)(b + 1899);
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| 102 |
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| 103 | if (g > 13.5)
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| 104 | *mn = (int)(g - 13);
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| 105 | if (*mn < 2.5)
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| 106 | *yr = (int)(b + 1900);
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| 107 | if (*yr < 1)
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| 108 | *yr -= 1;
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| 109 |
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| 110 | last_mn = *mn;
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| 111 | last_dy = *dy;
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| 112 | last_yr = *yr;
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| 113 | last_mj = mj;
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| 114 | }
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| 115 |
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| 116 | /* given an mjd, set *dow to 0..6 according to which day of the week it falls
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| 117 | * on (0=sunday).
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| 118 | * return 0 if ok else -1 if can't figure it out.
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| 119 | */
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| 120 | int
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| 121 | mjd_dow (double mj, int *dow)
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| 122 | {
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| 123 | /* cal_mjd() uses Gregorian dates on or after Oct 15, 1582.
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| 124 | * (Pope Gregory XIII dropped 10 days, Oct 5..14, and improved the leap-
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| 125 | * year algorithm). however, Great Britian and the colonies did not
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| 126 | * adopt it until Sept 14, 1752 (they dropped 11 days, Sept 3-13,
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| 127 | * due to additional accumulated error). leap years before 1752 thus
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| 128 | * can not easily be accounted for from the cal_mjd() number...
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| 129 | */
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| 130 | if (mj < -53798.5) {
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| 131 | /* pre sept 14, 1752 too hard to correct |:-S */
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| 132 | return (-1);
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| 133 | }
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| 134 | *dow = ((long)floor(mj-.5) + 1) % 7;/* 1/1/1900 (mj 0.5) is a Monday*/
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| 135 | if (*dow < 0)
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| 136 | *dow += 7;
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| 137 | return (0);
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| 138 | }
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| 139 |
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| 140 | /* given a year, return whether it is a leap year */
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| 141 | int
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| 142 | isleapyear (int y)
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| 143 | {
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| 144 | return ((y%4==0 && y%100!=0) || y%400==0);
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| 145 | }
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| 146 |
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| 147 | /* given a mjd, return the the number of days in the month. */
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| 148 | void
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| 149 | mjd_dpm (double mj, int *ndays)
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| 150 | {
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| 151 | static short dpm[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
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| 152 | int m, y;
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| 153 | double d;
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| 154 |
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| 155 | mjd_cal (mj, &m, &d, &y);
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| 156 | *ndays = (m==2 && isleapyear(y)) ? 29 : dpm[m-1];
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| 157 | }
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| 158 |
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| 159 | /* given a mjd, return the year and number of days since 00:00 Jan 1 */
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| 160 | void
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| 161 | mjd_dayno (double mj, int *yr, double *dy)
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| 162 | {
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| 163 | double yrd;
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| 164 | int yri;
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| 165 | int dpy;
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| 166 |
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| 167 | mjd_year (mj, &yrd);
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| 168 | *yr = yri = (int)yrd;
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| 169 | dpy = isleapyear(yri) ? 366 : 365;
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| 170 | *dy = dpy*(yrd-yri);
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| 171 | }
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| 172 |
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| 173 | /* given a mjd, return the year as a double. */
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| 174 | void
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| 175 | mjd_year (double mj, double *yr)
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| 176 | {
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| 177 | static double last_mj, last_yr;
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| 178 | int m, y;
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| 179 | double d;
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| 180 | double e0, e1; /* mjd of start of this year, start of next year */
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| 181 |
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| 182 | if (mj == last_mj) {
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| 183 | *yr = last_yr;
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| 184 | return;
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| 185 | }
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| 186 |
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| 187 | mjd_cal (mj, &m, &d, &y);
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| 188 | if (y == -1) y = -2;
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| 189 | cal_mjd (1, 1.0, y, &e0);
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| 190 | cal_mjd (1, 1.0, y+1, &e1);
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| 191 | *yr = y + (mj - e0)/(e1 - e0);
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| 192 |
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| 193 | last_mj = mj;
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| 194 | last_yr = *yr;
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| 195 | }
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| 196 |
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| 197 | /* given a decimal year, return mjd */
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| 198 | void
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| 199 | year_mjd (double y, double *mjp)
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| 200 | {
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| 201 | double e0, e1; /* mjd of start of this year, start of next year */
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| 202 | int yf = (int)floor (y);
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| 203 | if (yf == -1) yf = -2;
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| 204 |
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| 205 | cal_mjd (1, 1.0, yf, &e0);
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| 206 | cal_mjd (1, 1.0, yf+1, &e1);
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| 207 | *mjp = e0 + (y - yf)*(e1-e0);
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| 208 | }
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| 209 |
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| 210 | /* round a time in days, *t, to the nearest second, IN PLACE. */
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| 211 | void
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| 212 | rnd_second (double *t)
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| 213 | {
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| 214 | *t = floor(*t*SPD+0.5)/SPD;
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| 215 | }
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| 216 |
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| 217 | /* given an mjd, truncate it to the beginning of the whole day */
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| 218 | double
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| 219 | mjd_day(double mj)
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| 220 | {
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| 221 | return (floor(mj-0.5)+0.5);
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| 222 | }
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| 223 |
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| 224 | /* given an mjd, return the number of hours past midnight of the whole day */
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| 225 | double
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| 226 | mjd_hr(double mj)
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| 227 | {
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| 228 | return ((mj-mjd_day(mj))*24.0);
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| 229 | }
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| 230 |
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| 231 | /* insure 0 <= *v < r.
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| 232 | */
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| 233 | void
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| 234 | range (double *v, double r)
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| 235 | {
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| 236 | *v -= r*floor(*v/r);
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| 237 | }
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| 238 |
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| 239 | /* insure 0 <= ra < 2PI and -PI/2 <= dec <= PI/2. if dec needs
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| 240 | * complimenting, reflect ra too
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| 241 | */
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| 242 | void
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| 243 | radecrange (double *ra, double *dec)
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| 244 | {
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| 245 | if (*dec < -PI/2) {
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| 246 | *dec = -PI - *dec;
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| 247 | *ra += PI;
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| 248 | } else if (*dec > PI/2) {
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| 249 | *dec = PI - *dec;
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| 250 | *ra += PI;
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| 251 | }
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| 252 | range (ra, 2*PI);
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| 253 | }
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| 254 |
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| 255 | /* For RCS Only -- Do Not Edit */
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| 256 | static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: mjd.c,v $ $Date: 2005-08-21 10:02:38 $ $Revision: 1.5 $ $Name: not supported by cvs2svn $"};
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