Honestly though, the back of the Earth looks uncannily dark. I mean, this isn't the 18th century. So much nowadays is 24 x 7, especially our cities. Let's represent this with a separate Earth night texture that has city lights.
We have a file for you to use named earth_night_tex.jpg. Drag a copy of the file into your res/drawable/ folder.
It may be a little difficult to discern on this book's page, but this is what the texture image looks like:
To support this, we will create a new DayNightMaterial class that takes both versions of the Earth texture. The material will also incorporate the corresponding fragment shader that takes into consideration the normal vector of the surface relative to the light source direction (using dot products, if you're familiar with vector math) to decide whether to render using the day or night texture image.
In your res/raw/ folder, create files for day_night_vertex.shader and day_night_fragment.shader, and then define them...