Galileo is often credited with the early discovery of four of Jupiter's many moons. The moons orbiting Jupiter follow the same laws of motion as the planets orbiting the sun. One of the moons is called Io - its distance from Jupiter's center is 4.2 gigameters and it orbits Jupiter in 1.8 Earth-days. Another moon is called Ganymede; it is 10.7 gigameters from Jupiter's center. What is Ganymede's period in Earth days?