How can people be so sure that the earth is expanding? Also, if the Sun has an orbit what keeps it in its orbit?
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The Big Question 
The best way to think of the solar system and indeed the universe as a whole is not as planets orbiting stars, more the complex interaction between all of the gravitational fields generated by all of the bodies visible and not visible in the universe, including the now established presence of dark matter. To understand the nature of solar system orbits, one needs to consider the nature of gravity. Essentially gravity is a phenomenonally weak force that only objects with enormous mass generate however the force of gravity decreases rapidly with distance. Simply whilst all bodies in the solar system orbit our star, the forces of gravity from the planets etc do impact on each other, that being one of the reasons for eliptical aooner than circular orbits. As the the sun, the gravitational fields that impact upon it cause it to wobble as it rotates about its axis. It needs to also be mentioned that our entire solar system is rotating within our galaxy and that our galaxy is moving through the universe in a manner determine by the aforementioned gravitational fields that reach throughout the universe.