Orbital motions of astronomical bodies and their Centre of Mass from   different reference frames: a conceptual step between the Geocentric and   Heliocentric models
  The motion of astronomical bodies and the centre of mass of the system is not always well perceived by students. One of the struggles is the conceptual change of reference frame, which is the same that held back the acceptance of the Heliocentric model over the Geocentric one. To address the question, the notion of centre of mass, motion equations (and their numerical solution for a system of multiple bodies), and change of referential is introduced. The discussion is done based on conceptual and real world examples, using the solar system. Consequently, through the use of simple "do it yourself" methods and basic equations, students can debate complex motions, and have a wider and potentially effective understanding of physics. 
Andr\'e G C Guerra and Paulo Sime\~ao Carvalho
physics.ed-ph physics.pop-ph