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Diffstat (limited to 'time/equation_of_time.c')
-rw-r--r-- | time/equation_of_time.c | 86 |
1 files changed, 86 insertions, 0 deletions
diff --git a/time/equation_of_time.c b/time/equation_of_time.c new file mode 100644 index 0000000..a527c83 --- /dev/null +++ b/time/equation_of_time.c @@ -0,0 +1,86 @@ +/* + * (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: equation_of_time.c 2369 2013-04-28 14:19:35Z swfltek $ */ + +/* + The so called Equation of Time. + + The eccentricity of Earths orbit contributes about 7.7 minutes of variation to the result. It + has a period of 1 anomalous year, with zeroes at perihelion and aphelion. + + The tilt of Earths rotational axis (obliquity) contributes about 9.9 minutes of variation. It + has a period of 1/2 tropical year, with zeroes at solstices and equinoxes. The time of Earths + arrival at these events is influenced by the eccentricity, which causes it to progress along its + orbital path faster as it approaches perihelion, imposing a 'modulation' on the tropical phase. + + The algorithm employed computes the orbital position with respect to perihelion, deriving + from that a 'velocity correction factor'. The orbital position with respect to the winter solstice + is then computed, as modulated by that factor. The individual contributions of the obliquity and the + eccentricity components are then summed, and returned as an integer value in seconds. + +*/ + +#include <time.h> +#include <math.h> +#include "ephemera_common.h" + +int +equation_of_time(const time_t * timer) +{ + int32_t s, p; + double pf, sf, dV; + + /* compute orbital position relative to perihelion */ + p = *timer % ANOM_YEAR; + p += PERIHELION; + pf = p; + pf /= ANOM_CYCLE; + pf = sin(pf); + + /* Derive a velocity correction factor from the perihelion angle */ + dV = pf * DELTA_V; + + /* compute approximate position relative to solstice */ + s = *timer % TROP_YEAR; + s += SOLSTICE; + s *= 2; + sf = s; + sf /= TROP_CYCLE; + + /* modulate to derive actual position */ + sf += dV; + sf = sin(sf); + + /* compute contributions */ + sf *= 592.2; + pf *= 459.6; + s = pf + sf; + return -s; + +} |