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diff --git a/src/fragment.glsl b/src/fragment.glsl
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+// Copyright 2021, 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.
+
+#version 150
+
+// The color of sunlight (5800 K) in sRGB.
+const vec4 kSunColor = vec4(1.0, 0.9411765, 0.9137255, 1.0);
+
+in vec3 model_cartesian_position;  // unit vector pointing toward current point
+in vec2 texture_coordinate;
+
+uniform sampler2D planet;
+uniform sampler2D clouds;
+uniform vec3 sun_direction;  // unit vector pointing toward sun
+
+out vec4 out_color;
+
+void main() {
+  // Compute diffuse illumination from the Sun. We assume all rays are parallel,
+  // which isn't true, but at Earth scales, it causes a maximum error of about
+  // 0.003 degrees, well within our error budget.
+  float diffuse = max(dot(model_cartesian_position, sun_direction), 0.0);
+
+  // The planet is a mixture of the ground texture and the cloud texture. Assume
+  // clouds reflect sunlight; the ground texture needs no color adjustment,
+  // since it's already colored as if being hit by sunlight.
+  out_color = diffuse * mix(texture(planet, texture_coordinate), kSunColor,
+                            texture(clouds, texture_coordinate).r);
+}