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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: 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;
+
+}