/* * (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. */ /* $Id$ */ /* 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; }