[avr-libc-commit] [2360] Added code to account for the curvature of the terminator.

[Mike Rice]

Revision: 2360
          http://svn.sv.gnu.org/viewvc/?view=rev&root=avr-libc&revision=2360
Author:   swfltek
Date:     2013-04-24 22:22:58 +0000 (Wed, 24 Apr 2013)
Log Message:
-----------
Added code to account for the curvature of the terminator. This does nothing to 
improve the overall timing accuracy, but does improve yield a more natural 
illumination curve.

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

Modified: trunk/avr-libc/libc/time/moon_phase.c
===================================================================
--- trunk/avr-libc/libc/time/moon_phase.c       2013-04-23 22:32:18 UTC (rev 
2359)
+++ trunk/avr-libc/libc/time/moon_phase.c       2013-04-24 22:22:58 UTC (rev 
2360)
@@ -29,29 +29,36 @@
 /* $Id$ */
 
 /*
-    Return an approximation to the phase of the moon. This simple algorithm is 
almost as
-    accurate as more elaborate forumlae, given the limitations of the float 
implementation.
+    Return an approximation to the phase of the moon. Since no attempt is made 
to account for
+    Sol, Jupiter or Venus, it will often be off by several hours.
 */
 
 #include <time.h>
 
 int8_t
-moon_phase(time_t * gmt)
+moon_phase(time_t * timestamp)
 {
-    uint32_t        t;
-    int32_t         n;
+       uint32_t        t;
+       int32_t         n;
+       double          mc;
 
-    /* refer to first new moon of the epoch */
-    t = *gmt - 1744800UL;
+       /* refer to first new moon of the epoch */
+       t = *timestamp - 1744800UL;
 
-    /* constrain to 1 lunar cycle */
-    n = t % 2551443UL;
+       /* constrain to 1 lunar cycle */
+       n = t % 2551443UL;
 
-    /* offset by 1/2 lunar cycle */
-    n -= 1275721L;
+       /* offset by 1/2 lunar cycle */
+       n -= 1275721L;
+       mc = n;
+       mc /= 1275721.0;
+       mc *= M_PI;
+       mc = cos(mc) * sin(mc);
+       mc *= 12.5;
 
-    /* scale to range - 100...+ 100 */
-    n /= 12757L;
+       /* scale to range - 100...+ 100 */
+       n /= 12757L;
+       n -= mc;
 
-    return n;
+       return n;
 }