Who is orbiting whom? People usually think of our Sun as standing perfectly still, and Earth (and other planets) revolve around it.
Yet both the Earth and Sun have mass. Both attract each other. For every force, there is an equal and opposite force (3rd law)
Thus the Earth (and other planets) pull on the Sun.
Wrong idea: The Sun stands still and the Earth revolves around it.
Better idea: The Sun and Earth mutually rotate around their center-of-gravity. This special place that they rotate around is called the barycenter.
Etymology: from the ancient Greek βαρύς heavy κέντρον center.
agron-www.agron.iastate.edu/courses/Agron541/classes/541/lesson07b/7b.2.html
The barycenter is the center of mass of two or more bodies that orbit one another; it is the point about which they orbit.
This fact is very important in astronomy and astrophysics.
How do we observe this in our own solar system? We see that the motion of our Sun has a wobble.
Since the Sun is much more massive than Earth, the center of mass is closer to the Sun.
The center of mass, in fact, is inside the diameter of the Sun, so the resulting wobble caused by Earth is small.
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What would this same system look like, as seen from the side?
What would this look like, if 2 objects of similar size orbited around each other? The Pluto–Charon system
The Sun wobbles even more than this due to the pull from other worlds. Jupiter has a large mass, it causes a big wobble.
Learning Standards
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
8.MS-ESS1-2. Explain the role of gravity in ocean tides, the orbital motions of planets, their moons, and asteroids in the solar system.
HS-PS2-4. Use mathematical representations of Newton’s law of gravitation and Coulomb’s law to both qualitatively and quantitatively describe and predict the effects of gravitational and electrostatic forces between objects.
Next Generation Science Standards
HS-PS2.B.1 ( High School Physical Sciences ): Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects.
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