Eistein’s general relativity theory is indeed a groundbreaking theory in physics and astronomy.
Considering, a planet is orbiting a star.
Generally, we look at them like this, from top down.
As you can see, the small planet orbits around the star clockwise. But that is set the frame of reference at the center of the star. Or the center of mass of the star.
If you set the frame of reference at the planet:
It looks like the star is orbiting around the planet. Because picture that you’re on that planet, and you see that star. Because you’re relatively not moving, now the star is moving.
So if we draw a overall picture, it looks like this.
As the frames of reference are different, the orbits are different. However, the radiuses of the two orbits are the same. And no matter how you set the frame of reference, it’s still clockwise orbiting.
So now comes the question, sine we already know how object moves if we are ON either object, or AS either object. What does the movement of this mini solar system look like if we are OUT of the two objects?
And we can set a new frame of reference.
It was you and me, and now it is he or she. The third party looks at things always differently.
As we can see, if we are outside of the orbits, and set the frame of reference to us, the outside observer.
The movement of the planet and the star looks like they are revolving together along the line of that center circle.
And they all orbit around the center of that circle. And that circle, is a much smaller circle than the other two circles, on which object we set the frame of reference.
Picture this, you’re out on a walk, and you see two dogs chasing each other around. They chase each other’s tail like a circle.
You, are the observer of the movements of these two dogs. What if, you were one of the dogs? If you were Pietro (the big dog), what did Jacob (the smaller dog) look like when you were on his tail?
And picture you were Jacob, what did Pietro move like when you were on his tail?
Out of the lovely play of the two dogs, we, the observers, simply look at them as spinning around the center of each other. That radius is drawn from the circle of both of their movements, but not like the orbits that put on each of the objects.
Awesome, isn’t it?