moon, official name

This is actually a name followed by two numbers.  The first part is the official name of the planet which the moon goes round.  Earth, for instance, is Sol 3.  You make the moon’s name by adding a number – counting the moons out from the planet. Earth has only one moon, so it’s Sol 3-1.  Ganymede, the largest moon of Jupiter (Sol 5) is the seventh nearest, so its official name is Sol 5-7.