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.