The Planet Saturn is the 6th from the sun.

Some uninterresting informations:

Saturn is a very interresting planet. With its mystifying ring, Saturn is the second biggest Planet after Jupiter (Sailor Jupiter is the taallest, hum? But Sailor Saturn isn't the second tallest, not at all!) Its density is so low that it would float if it was placed in water.

"Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christian Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly thereafter around Jupiter and Neptune). " (Saturn)

Saturn has a lot of moons, and one of the biggest of all is called Titan (it is the fourth biggest). Doesn't it remember you Saturn's Castle? Titan's number is VI. And Saturn is the 6th from the moon. All these things can explain Hotaru's birth date (january 6th). Titan had been the first moon discovred of Saturn's moons, in 1655; this is why it is one the the most known of Saturn's moons. This is also why, in my opinion, Naoko Takeuchi took this name for Hotaru's Castle.