Watch two bodies orbit their common centre of mass (marked with a +). This is a top-down view of a gravitational system. Both bodies move — the heavier one moves less, the lighter one moves more.
Units: Mass is measured in solar masses (M☉), where 1 M☉ = the mass of our Sun (≈ 2 × 10³⁰ kg). Distance is measured in astronomical units (AU), where 1 AU = Earth-Sun distance (≈ 150 million km).