Unity3d Learning rotate and rotatearound do the Earth revolve around the sun

Video Teaching Source: www.51cto.com

New 3 sphere, named Sun,earth,moon,

New 2 C#script, named ROTATESCR,ROTATEAROUNDSCR

Rotatescr writes only one sentence in update ()

This.transform.Rotate (Vector3.up*1f,space.world);

Rotatearoundscr also wrote a sentence in Upate ()

This.transform.RotateAround (rotatearoundobj.position,vector3.up,1f);

The public Transform Rotatearoundobj is the object that is being rounded.
Drag ROTATESCR to 3 sphere respectively,

Drag rotatearoundscr to Earth and moon

Finally, the sun dragged to the Earth's rotatearoundscr in the Rotatearoundobj, the earth dragged to moon Rotatearoundscr rotatearoundobj.

At the beginning of the beginner stage, only know how to do, as for why, later deepen understanding.