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I am aware of this Phys.SE question: Why do we say that the earth moves around the sun? but I don't think this is a duplicate.

In a binary star system, where the masses of the 2 stars are not so different from each other, can we say that each star revolves around the other?

If yes, then couldn't the Sun-Earth system be an extreme case of such a system? Therefore, strictly speaking, can we argue that the Sun revolves a tiny bit around the Earth as well?

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    $\begingroup$ In a binary star system, where the masses of the 2 stars are not so different from each other, can we say that each star revolves around the other? I would say that they revolve around the center of mass. $\endgroup$ – jinawee Mar 6 '15 at 19:55
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In binary systems, each object is so affected by the others gravity that they have significant orbit. The sun has so much inertia that the earth's pull barely moves it, but the earth certainly revolves around the sun. In the reference frame of the Earth however, the Sun does revolve around the Earth.

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  • $\begingroup$ So, strictly speaking, the Sun too revolves around the Earth, it's just a matter of intensity? $\endgroup$ – benji Mar 6 '15 at 20:01
  • $\begingroup$ It is not a matter of intensity. It is a matter of reference frames. Imagine this: If you stood on the sun, you would see the e $\endgroup$ – Jimmy360 Mar 6 '15 at 20:02
  • $\begingroup$ arth revolving, but on the Earth you see the Sun revolving, Einstien showed us that both views are correct, just as in a car, you seem still and everything else is moving. $\endgroup$ – Jimmy360 Mar 6 '15 at 20:03
  • $\begingroup$ @ben I suggest you to read my answer too. The Earth was considered as orbiting the Sun after considering the movement of the rest of the celestial bodies. $\endgroup$ – Sofia Mar 6 '15 at 20:14
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Both the the Earth and the Sun orbit around the solar system barycentre. This is defined as the centre of mass of all the bodies in the solar system. Because the Sun contains the vast majority of the mass of the solar system then the barycentre is very close to the Sun. The picture below, from the wikipedia entry on the solar system barycentre, has the barycentre stationary in the middle of the picture, and illustrates schematically the situation when one body is a lot more massive than the other (though is not to scale for the Earth-Sun system!).

The Sun executes a complicated orbit around this point (also illustrated here), pulled by the motions of, primarily Jupiter, but all the other planets also make a smaller contribution.

It is this "reflex motion" of a star, caused by planets in its solar system, that allows the detection of exoplanets by the doppler method. The orbit of the Sun around the barycentre would cause it to appear to a distant observer to be periodically redshifted and blue shifted with an amplitude of about 13 m/s, with a period of around 12 years (the orbital period of Jupiter). In isolation, the Earth would only cause the Sun to orbit the barycentre of the Earth-Sun system with a speed of 7 cm/s (which is one reason that finding Earth-like exoplanets is very difficult).

Reflex motion of a star

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There are at least 8 more planets in the solar system, besides the Earth (and some more were discovered). When Copernicus decided to place the Sun in the center of the solar system, instead of the Earth, that was mainly because this arrangement simplified drastically the form of the orbits of the other planets. With the Geocentric model of the solar system those orbits appeared as very complicated.

Later on, the Kant-Laplace theory for the solar system, strengthened this Heliocentric configuration.

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  • $\begingroup$ That's exactly the right point. We can describe motion of planets etc in any frame of reference; but the advantage of describing the motion in the frame of reference connected with the Sun or, to be exact, with the center of mass of the solar system is that the description is simplified a lot. If simplicity of description is the same as the truthfulness of it then it is true that the Earth moves around the Sun. $\endgroup$ – Maxim Umansky Mar 1 '17 at 21:41

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