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This is a diagram displaying the centre of mass. This is displayed as a blue circle in the centre. There are double-ended arrows going from the centre, either side, in the direction of the top left and bottom right corners of the image. The arrow going towards the top left of the image is labelled aplanet and the other arrow is labelled astar. There are two circles surrounding the centre of mass, they both have arrows going in an anticlockwise direction. Underneath this, there is the label: aplanet / astar = Mstar / Mplanet.

 3.1  Gravity, orbits and see-saws