Supplemental: Astronomy

How Haruka and Michiru relate to their respective planets.

Sailor Uranus is originally the princess of the planet Uranus. During the Silver Milennium, Queen Serenity of old granted her a castle to call her own. This palace, Miranda Castle, orbits Uranus and is named after the smallest of Uranus' five innermost satellites. Likewise, Sailor Neptune is the princess of Neptune and was given Triton Castle as her birthright. Triton Castle is named after the largest satellite orbiting Neptune. Both castles orbit their respective planets.

Both Uranus and Neptune are very similar in composition. Although they are gaseous like Jupiter and Saturn, their interiors are composed mostly of ice and rock, making them ice giants. These two planets, much like the soldiers that represent them, stand out among the rest of the planets as a very unique pair.

Interestingly enough, although Sailor Uranus has strong ties to the wind, Neptune harbors the strongest winds in the solar system with the highest recorded speed at 2,100 km. Neither planet has any solid evidence of oceans, although scientists do believe there is a possibility that both may have them far down below their atmospheric surfaces. Instead, Sailor Uranus and Sailor Neptune both have powers based on their Roman/Greek namesakes.

Time from time, we see both Sailor Uranus and Sailor Neptune obtain a glowing symbol on their forehead. These symbols are often used in imagery and stationery featuring the two soldiers. It should come as no surprise that these symbols are the actual astronomical symbols used to represent the planets. Uranus' symbol, a globe surrounded by the letter H, was created as a tribute to the man who originally found Uranus (William Herschel). Neptune's symbol, a trident, is representative of the weapon often wielded by the Roman god of the same name.

Colour-wise, Sailor Uranus' blue uniform is a reference to the blue colour of Uranus. Likewise, Neptune's deeper blue accent colour relates to Neptune's darker blue hues.

Lastly, even in reincarnated civilian form, Tenoh Haruka and Kaioh Michiru get their names from their respective planets as the Japanese name for Uranus is "Tenousei" and Neptune is "Kaiousei."

For further reading about both planets, I recommend checking out Wikipedia's articles on Uranus and Neptune.