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diff --git a/src/solar.h b/src/solar.h
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+/**
+ * Copyright © 2016  Mattias Andrée <maandree@member.fsf.org>
+ * 
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ * 
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+
+/**
+ * Approximate apparent size of the Sun in degrees.
+ */
+#define LIBRED_SOLAR_APPARENT_RADIUS  (32.0 / 60.0)
+
+
+/**
+ * The Sun's elevation at sunset and sunrise, measured in degrees.
+ */
+#define LIBRED_SOLAR_ELEVATION_SUNSET_SUNRISE   (0.0)
+
+/**
+ * The Sun's elevation at civil dusk and civil dawn, measured in degrees.
+ */
+#define LIBRED_SOLAR_ELEVATION_CIVIL_DUSK_DAWN   (-6.0)
+
+/**
+ * The Sun's elevation at nautical dusk and nautical dawn, measured in degrees.
+ */
+#define LIBRED_SOLAR_ELEVATION_NAUTICAL_DUSK_DAWN  (-12.0)
+
+/**
+ * The Sun's elevation at astronomical dusk and astronomical dawn, measured in degrees.
+ */
+#define LIBRED_SOLAR_ELEVATION_ASTRONOMICAL_DUSK_DAWN  (-18.0)
+
+
+/**
+ * Test whether it is twilight.
+ * 
+ * @param   ELEV:double  The current elevation.
+ * @return               1 if is twilight, 0 otherwise.
+ */
+#define LIBRED_IS_TWILIGHT(ELEV)  ((-18.0 <= (ELEV)) && ((ELEV) <= 0.0))
+
+/**
+ * Test whether it is civil twilight.
+ * 
+ * @param   ELEV:double  The current elevation.
+ * @return               1 if is civil twilight, 0 otherwise.
+ */
+#define LIBRED_IS_CIVIL_TWILIGHT(ELEV)  ((-6.0 <= (ELEV)) && ((ELEV) <= 0.0))
+
+/**
+ * Test whether it is nautical twilight.
+ * 
+ * @param   ELEV:double  The current elevation.
+ * @return               1 if is nautical twilight, 0 otherwise.
+ */
+#define LIBRED_IS_NAUTICAL_TWILIGHT(ELEV)  ((-12.0 <= (ELEV)) && ((ELEV) <= -6.0))
+
+/**
+ * Test whether it is astronomical twilight.
+ * 
+ * @param   ELEV:double  The current elevation.
+ * @return               1 if is astronomical twilight, 0 otherwise.
+ */
+#define LIBRED_IS_ASTRONOMICAL_TWILIGHT(ELEV)  ((-18.0 <= (ELEV)) && ((ELEV) <= -12.0))
+
+
+
+/**
+ * Calculates the Sun's elevation as apparent.
+ * from a geographical position.
+ * 
+ * @param   latitude   The latitude in degrees northwards from 
+ *                     the equator, negative for southwards.
+ * @param   longitude  The longitude in degrees eastwards from
+ *                     Greenwich, negative for westwards.
+ * @return             The Sun's apparent elevation as seen, right now,
+ *                     from the specified position, measured in degrees.
+ * 
+ * @throws  0  On success.
+ * @throws     Any error specified for clock_gettime(3) on error.
+ */
+double libred_solar_elevation(double latitude, double longitude);
+
+
+/**
+ * Exit if time the is before year 0 in J2000.
+ * 
+ * @return  0 on success, -1 on error.
+ */
+#if defined(TIMETRAVELLER)
+int libred_check_timetravel(void);
+#else
+# define libred_check_timetravel()  0
+#endif
+