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+// Copyright 2022 Benjamin Barenblat
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+#ifndef GLPLANET_SRC_ASTRO_H_
+#define GLPLANET_SRC_ASTRO_H_
+
+#include <chrono>
+#include <utility>
+
+#include "third_party/date/include/date/tz.h"
+
+namespace glplanet {
+
+// The apparent right ascension and declination of the sun, in radians. Accurate
+// to 0.01 degrees.
+std::pair<double, double> SunEquatorialPosition(
+    std::chrono::time_point<date::tai_clock,
+                            std::chrono::milliseconds>) noexcept;
+
+// The apparent sidereal time at Greenwich, in radians.
+double ApparentSiderealTimeAtGreenwich(
+    std::chrono::time_point<date::tai_clock,
+                            std::chrono::milliseconds>) noexcept;
+
+// The east longitude and north latitude of the location observing a Sun transit
+// (i.e., the location at which it's high noon), in radians.
+std::pair<double, double> HighNoonLocation(
+    std::chrono::time_point<date::tai_clock,
+                            std::chrono::milliseconds>) noexcept;
+
+}  // namespace glplanet
+
+#endif  // GLPLANET_SRC_ASTRO_H_