Building an earth-moon system

Let's add the moon and animate it to orbit the earth. We will organize this in the hierarchy under a new container object that we'll call Earth-Moon. This way, we can keep the earth and the moon together so later we can move them as a unit (a celestial pair) when they orbit around the sun.

You may wonder why not just make the moon a child object of the earth so they'll move together around the sun? Well, the earth also spins its day-night cycle, and we don't want the moon to inherit that angular rotation, so its orbit is separate.

Creating the container object

First, let's create the Earth-Moon container:

1. In Hierarchy, create a new empty object under SolarSystem, named Earth-Moon and reset its Transform.
2. Move Earth as a child of Earth-Moon.
3. Set their positions as follows: SolarSystem (0, 0.75, 0), Earth-Moon (0, 0, 0), and Earth (0, 0, 0).

The resulting hierarchy is shown in the following screenshot:

Creating the moon

Next, we can create a Moon object from sphere. We...