We often study orbits where one mass is many times greater than the other mass. For example, the sun's mass is 333,000 times more than the earth's mass, and the earth's mass is 81 times more than the moon's mass. Such orbits create the impression that one mass is stationary, while the other revolves around. In fact, the objects revolve around each other, like dancers holding hands and spinning around each other. The more massive dancer simply moves in smaller circles.
Newton's first law told us that an object in motion will continue moving in a straight line, unless acted upon by an external force. He was speaking of objects that were treated as a point. Euler later extended Newton's law to deal with a system of objects. The center of mass of a system of objects will move in a straight line, unless acted upon by an external force.
The simulator below puts you in charge of specifying two masses in circular orbits with a given separation. The default values are for the earth and the moon. Think of some questions, and try to answer them with the simulator.
Here is the accompanying handout.