[avr-libc-commit] [2336] Edited equation_of_time.

[Mike Rice]

Revision: 2336
          http://svn.sv.gnu.org/viewvc/?view=rev&root=avr-libc&revision=2336
Author:   swfltek
Date:     2013-04-07 16:09:23 +0000 (Sun, 07 Apr 2013)
Log Message:
-----------
Edited equation_of_time. Separating the tropical and anomalous portions 
provided several more bits to work with.

Modified Paths:
--------------
    trunk/avr-libc/libc/time/equation_of_time.c

Modified: trunk/avr-libc/libc/time/equation_of_time.c
===================================================================
--- trunk/avr-libc/libc/time/equation_of_time.c 2013-04-06 19:23:16 UTC (rev 
2335)
+++ trunk/avr-libc/libc/time/equation_of_time.c 2013-04-07 16:09:23 UTC (rev 
2336)
@@ -29,52 +29,53 @@
 /* $Id$ */
 
 /*
-       The so called Equation of Time. We determine the orbital position of 
the mothership,
-       and solve a two term polynomial. More accurate equations are available 
(more terms), but
-       without a true double there is little point in that.
+    The so called Equation of Time.
 */
 
 #include <time.h>
 #include <math.h>
 
+#define TROP_YEAR 31556925
+#define ANOM_YEAR 31558433
+#define PERIHELION 31218166 /* perihelion of 1999 */
+#define SOLSTICE 836160 /* winter solstice of 1999 */
+#define TWO_PI 6.283185307179586
+
 int
 equation_of_time(time_t * timer)
 {
-       unsigned long   s, p, y;
-       double          d, lf, hf;
+    unsigned long   s, p, y;
+    double          d, lf, hf;
 
-       /* s = time since solstice */
-       s = *timer + 836160UL;
+    /* s = time since solstice */
+    s = * timer % TROP_YEAR;
+    s += SOLSTICE;
 
-       /* p = time since perihelion */
-       p = *timer - 191857UL;
+    /* p = time since perihelion */
+    p = *timer % ANOM_YEAR;
+    p += PERIHELION;
 
-       /* limit to 1 tropical year */
-       y = 31556926;
-       s %= y;
-       p %= y;
+    /* low frequency component has 1 year period */
+    lf = p;
+    lf /= ANOM_YEAR;
 
-       /* low frequency component has 1 year period */
-       lf = p;
-       lf /= y;
+    /* high frequency component has 1/2 year period */
+    y = TROP_YEAR / 2;
+    hf = s;
+    hf /= y;
 
-       /* high frequency component has 1/2 year period */
-       y /= 2;
-       hf = s;
-       hf /= y;
+    /* convert years to radians */
+    d = 6.283185307179586;
+    hf *= d;
+    lf *= d;
 
-       /* convert years to radians */
-       d = 6.283185307179586;
-       hf *= d;
-       lf *= d;
+    /* hf has magnitude 592.2 */
+    hf = sin(hf) * 592.2;
 
-       /* hf has magnitude 592.2 */
-       hf = sin(hf) * 592.2;
+    /* lf has magnitude 451.8 */
+    lf = sin(lf) * 451.8;
 
-       /* lf has magnitude 451.8 */
-       lf = sin(lf) * 451.8;
+    d = lf + hf;
 
-       d = lf + hf;
-
-       return -d;
+    return -d;
 }