Ganymede (moon) Ganymede (moon) Ganymede (Jupiter III) is the largest and most massive moon of Jupiter and in the Solar System. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. It is the third of the Galilean moons, the first group of objects discovered orbiting another
Orbit of the Moon orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity). As was discovered by Jacques Cassini in 1722, the rotational axis of the Moon precesses with the same rate as its orbital plane, but is 180° out of phase "(see Cassini's Laws)". Therefore, the angle between the ecliptic and the lunar equator is always 1.543°, even though the rotational axis of the Moon is not fixed with respect to the stars. The nodes are points at which the Moon's orbit crosses the ecliptic. The Moon crosses the same node every 27.2122 days, an interval