or at the centre of the earth, a distance of about 6,300km. Body Diameter Semi-major axis Sun (215 mm) zero Mercury (0.8 mm) (9 m) Venus (1.9 mm) (17 m) Earth (2 mm) (23 m) Mars (1 mm) (35 m) Ceres (0.2 mm) (64 m) Jupiter (21.6 mm) (120 m) Saturn