1 files changed, 65 insertions, 0 deletions
diff --git a/time/moon_phase.c b/time/moon_phase.c
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+/*
+ * (C)2012 Michael Duane Rice All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer. Redistributions in binary
+ * form must reproduce the above copyright notice, this list of conditions
+ * and the following disclaimer in the documentation and/or other materials
+ * provided with the distribution. Neither the name of the copyright holders
+ * nor the names of contributors may be used to endorse or promote products
+ * derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/* $Id: moon_phase.c 2365 2013-04-27 15:32:59Z swfltek $ */
+
+/*
+    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>
+#include <math.h>
+
+int8_t
+moon_phase(const time_t * timestamp)
+{
+	uint32_t        t;
+	int32_t         n;
+	double          mc;
+
+	/* refer to first new moon of the epoch */
+	t = *timestamp - 1744800UL;
+
+	/* constrain to 1 lunar cycle */
+	n = t % 2551443UL;
+
+	/* 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;
+	n -= mc;
+
+	return n;
+}