From 7c1bc2ff6bd1a46f3587234928e8bffc76102294 Mon Sep 17 00:00:00 2001
From: "C. J. Howard" <cjhoward@cjhoward.org>
Date: Thu, 20 May 2021 00:19:52 +0800
Subject: [PATCH] Add physics::time namespace, add function to calculate ERA
 from UT1

---
 src/ecs/systems/astronomy-system.cpp |  3 +-
 src/physics/physics.hpp              |  1 +
 src/physics/time/time.hpp            | 32 +++++++++++++++++
 src/physics/time/ut1.hpp             | 51 ++++++++++++++++++++++++++++
 4 files changed, 86 insertions(+), 1 deletion(-)
 create mode 100644 src/physics/time/time.hpp
 create mode 100644 src/physics/time/ut1.hpp

diff --git a/src/ecs/systems/astronomy-system.cpp b/src/ecs/systems/astronomy-system.cpp
index 3dd6d23..9614e39 100644
--- a/src/ecs/systems/astronomy-system.cpp
+++ b/src/ecs/systems/astronomy-system.cpp
@@ -24,6 +24,7 @@
 #include "renderer/passes/sky-pass.hpp"
 #include "color/color.hpp"
 #include "physics/orbit/orbit.hpp"
+#include "physics/time/ut1.hpp"
 #include "geom/cartesian.hpp"
 #include <iostream>
 
@@ -122,7 +123,7 @@ void astronomy_system::update(double t, double dt)
 			
 			
 			const double earth_axial_tilt = math::radians(23.45);
-			const double earth_axial_rotation = math::two_pi<double> * (0.7790572732640 + 1.00273781191135448 * universal_time);
+			const double earth_axial_rotation = physics::time::ut1::era(universal_time);
 			const double earth_radius_au = 4.2635e-5;
 			
 			const double observer_altitude = earth_radius_au;
diff --git a/src/physics/physics.hpp b/src/physics/physics.hpp
index 4f04a13..bf71126 100644
--- a/src/physics/physics.hpp
+++ b/src/physics/physics.hpp
@@ -25,5 +25,6 @@ namespace physics {}
 
 #include "frame.hpp"
 #include "orbit/orbit.hpp"
+#include "time/time.hpp"
 
 #endif // ANTKEEPER_PHYSICS_HPP
diff --git a/src/physics/time/time.hpp b/src/physics/time/time.hpp
new file mode 100644
index 0000000..8f96d7e
--- /dev/null
+++ b/src/physics/time/time.hpp
@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2021  Christopher J. Howard
+ *
+ * This file is part of Antkeeper source code.
+ *
+ * Antkeeper source code 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.
+ *
+ * Antkeeper source code 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 Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ANTKEEPER_PHYSICS_TIME_HPP
+#define ANTKEEPER_PHYSICS_TIME_HPP
+
+namespace physics {
+
+/// Time-related functions.
+namespace time {}
+
+#include "ut1.hpp"
+
+} // namespace physics
+
+#endif // ANTKEEPER_PHYSICS_TIME_HPP
diff --git a/src/physics/time/ut1.hpp b/src/physics/time/ut1.hpp
new file mode 100644
index 0000000..e91560d
--- /dev/null
+++ b/src/physics/time/ut1.hpp
@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2021  Christopher J. Howard
+ *
+ * This file is part of Antkeeper source code.
+ *
+ * Antkeeper source code 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.
+ *
+ * Antkeeper source code 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 Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ANTKEEPER_PHYSICS_TIME_UT1_HPP
+#define ANTKEEPER_PHYSICS_TIME_UT1_HPP
+
+#include "math/constants.hpp"
+
+namespace physics {
+namespace time {
+
+/// Functions which operate on UT1 universal time.
+namespace ut1 {
+
+/**
+ * Calculates the Earth Rotation Angle (ERA) at a given UT1 date.
+ *
+ * @param t J2000 UT1 date.
+ * @return ERA at the given date, in radians.
+ */
+template <class T>
+T era(T t);
+
+template <class T>
+T era(T t)
+{
+	return math::two_pi<T> * (T(0.7790572732640) + T(1.00273781191135448) * t);
+}
+
+} // namespace ut1
+
+} // namespace time
+} // namespace physics
+
+#endif // ANTKEEPER_PHYSICS_TIME_UT1_HPP