The senshi's names, birthdays, and their powers and attacks all relate to their planets.  Also, the days of the week are each named after planets.  This part of the page will explain about all that stuff, as well as some information on mythology and things like that.
Selene is the Greek goddess of the moon.  Her Roman counterpart is Luna.  Selene is probably where the name Serenity came from, or from the Sea of Serenity on the moon.

Endymion was a shepherd in a Greek myth.  Selene fell in love with him and put him into eternal sleep so she could watch over him.  He also sired 50 of her daughters (while he was sleeping).

Artemis was the Greek goddess of the hunt, chastity, and fertility.  Her counterpart in Roman mythology was Diana.  She was the daughter of Zeus (Jupiter to the Romans).  She was also associated with the lunar cycle and the moon.

Mercury (Hermes to the Greeks) was Jupiter's messenger.  He was also the god of storytelling, eloquence, commerce, and chance.  According to the mythology, Mercury was the son of Jupiter and Maia, and on noon on the day of his birth he invented the lyre (notice that Sailor Mercury uses a harp in her attack).  Hermes was the god of good luck, wealth, commerce, sleep, and dreams.  He conducted souls of the dead to Hades, and was the messenger of the gods who flew around on winged sandals.  He was also the patron god of thieves, merchants, and deception.

Venus was the daughter of Jupiter and a Titan, also known as Aphrodite to the Greeks.  At birth she emerged from sea foam.  She was the goddess of love, charm, and beauty.  In the mythology, Jupiter (Zeus) married her, the most beautiful goddess, to Vulcan (Hephaestus), the ugliest god, even though Jupiter hated Vulcan (to me that makes no sense at all).  Venus is also the mother of Cupid (Eros) and was in love with Mars (Ares), and she also had a sister, Eris, goddess of discord.

Mars (Ares in Greek) was the god of war, associated with military power and glory.  He spent his whole life battling and fighting and making war.  He was hated by the other gods, except for Venus and Eris.

Jupiter (also known as Jove, or Zeus in Greek) was the god of the heavens and earth, basically the father of all the gods, and god of justice.  He was the son of Saturn and Rhea.  When he was old enough he overthrew his father.  Jupiter was the god of the sky, thunder, and rain.

Saturn was the god of the harvest and the father of most of the other Roman gods, known as Cronus to the Greeks.  He was the son of Uranus and Gaea, and also the god of time.  He and his sister Rhea had six children, but Cronus ate the first five.  Rhea saved the sixth, Zeus (Jupiter), and Zeus eventually overthrew Cronus.

Uranus was the personification of the stars and the sky.  He was spontaneously born from the earth, and he was the husband and son of Gaea.  Their children were the Titans, the Cyclops, the three-hundred-handed monsters, and some other weird creatures.  Uranus is also said to be an androgynous planet.

Neptune was the Roman god of water, rain, and fertility, counterpart to the Greek god Poseidon.  Originally the god of earthquakes and water, but eventually became the god of the sea.  He was also the god of horses, because he is said to have created them.  Neptune was also the patron god of homosexuals.

Pluto is the god of the Underworld, counterpart to the Greek Hades.  He was connected with a being called Plutus, who personifies wealth.  There wasn't that much known about this god (just like there isn't much known about Sailor Pluto either)
Days of the week
Maybe you didn't know this, but each day of the week is named after a planet.  Below are the days of the week and how they correspond to the planets (this isn't really Sailormoon stuff, but you've gotta admit it's interesting).  Most of this info came from Sailor FlowerPower's Moon Garden, except for the info about the Spanish days, that I came up with myself. *g*

The day, Monday, is connected to the moon.  The Japanese name for the day Monday is Getsuyoubi, meaning Moon Day (getsu is another way to pronounce the kanji for tsuki).  In Old English Monday is called Monendai, and in Latin it is called dies Lunae, both of those names meaning Moon Day.  In Spanish it is called Lunes, which is derived from Luna, the Spanish name for the moon (and also Usagi's guardian cat).  Plus, if you add one o to the word Monday, it becomes Moonday.

The day Tuesday is connected to Mars (this one's less obvious).  The Japanese name for Tuesday is Kayoubi, which means fire day (and the planet Mars is called Kasei).  The Norse name for Tuesday is Tiwesdaeg, which means day of Tiw, and Tiw was the god of war.  In Latin, Tuesday is called dies Martis, which means day of Mars (Mars was the god of war, like Tiw to the Norse).  In Spanish, Tuesday is called Martes.  Marte means Mars in Spanish.

Wednesday is connected to Mercury.  It is called Wodnesdaeg in Norse.  Wodnesdaeg means day of Woden, Woden being the god of wisdom and also the king of the gods in Norse mythology (that I guess explains why Mercury's so smart).  In Japanese, Wednesday is called Suiyoubi, which means water day (the planet Mercury is called suisei).  In Spanish the day Wednesday is called can kind of see the word Mercury in Miercoles, can't you?  Mercury is also the god of commerce, and the patron god of thieves (hehehe).

Thursday is connected to Jupiter.  In Norse it is called Thuresdaig, which means day of Thor.  Thor was the god of storms, lightning, and thunder.  In Latin it is called dies Jovis, which means day of Jove.  Jove is another name for Jupiter, who was the god of the sky, lightning, and thunder (Jupiter's attacks = lightning), as well as king/father of the gods.  In Japanese, this day is called Mokuyoubi, which means wood day (and Jupiter is called Mokusei, wood star).  In Spanish, Thursday is called Jueves, which to me looks like Jove.

Friday is connected to Venus.  In Japanese, Friday is called kin'youbi, which means gold day (Venus in Japanese = kinsei, gold star).  In Norse, Friday was called Frigedaeg (meaning day of Freya, who was the goddess of love and beauty in Norse).  In Latin it was called dies Veneria, which means day of Venus, who was the goddess of love and beauty (Venus Love and Beauty Shock, Sailor Venus's attack).  In Spanish, Friday is called Viernes, and can't you see the word Venus in Viernes?

Saturday is connected to Saturn (no, really? *sarcasm*).  In Japanese, Saturday is called Doyoubi (earth day, also connected to dosei, the planet Saturn).  In Norse it was called Saturnedai, which means day of Saturn.  In Latin it was called dies Saturni (also meaning day of Saturn, obviously).  The Roman god Saturn was a god of the earth (you can see the connection to the Japanese name).  In Spanish, Saturday is called Sabado (I think that sounds like Saturn, very loosely), which is connected to the Sabbath (seventh day).

Obviously, Sunday is connected to the sun.  The word sun is in the day name.  In Spanish, Sunday is called Domingo, which is derived from the Latin word Dominus (our Lord).
Zodiac Signs
The senshi's zodiac signs are astrologically correct with the planets they represent.  See, each astrological sign has a certain planet that it is associated with, and an element as well.  Here are the senshi and their zodiac signs, and what they're supposed to mean...

Planet associated with: Mars
Element: fire
Comments: Hino Rei is an Aries, and if you look in a book or something about astrological stuff, Rei fits the description for Aries too.  Energetic, impulsive, extroverted...

Planet associated with: Venus
Element: earth

Planet associated with: Mercury
Element: air

Planet associated with: the moon
Element: water
Comments: Usagi and Chibiusa are both Cancers, born on the same day too (June 30).  They kind of fit the personality description for Cancers, not to the letter of course, but sort of.

Planet associated with: the sun
Element: fire
Comments: Tuxedo Kamen is a Leo, August 3rd.  But...he so does not fit the description.  Extroverted, spotlight-seeker?  Not Tux.

Planet associated with: Mercury
Element: earth
Comments: Ami is a Virgo, and she's sensible, logical, a perfectionist, introverted, and all the other things that Virgos are supposed to be.

Planet associated with: Venus
Element: air
Comments: Minako is a Libra, and as for her element, she is an airhead... Just kidding.  She sorta fits the stereotypical Libra description: extroverted, sociable, charming...that's about it though.

Planet associated with: Pluto
Element: water
Comments: Scorpios are supposed to be somewhat closed and mysterious (which I can attest to, being one myself).  Setsuna is a Scorpio, I think she fits the stereotype.

Planet associated with: Jupiter
Element: fire
Comments: Makoto, a Saggitarius, fits the stereotype for Saggitarians quite well: optimistic, friendly, casual, direct, outspoken, freedom-seeking, honest, loyal, etc.

Planet associated with: Saturn
Element: earth
Comments: Hotaru is a Capricorn, since she was born on January 6th.  But she doesn't fit the stereotype: principled, stingy, business oriented and political?  That doesn't sound very Hotaru-type to me, she's more like a Scorpio.  Oh yeah, and the element for Capricorns is earth, that's also the element for the planet Saturn.

Planet associated with: Uranus
Element: air
Comments: Haruka is an Aquarius who is individualistic, independent, detatched, non-conforming, perceptive, and logical, like the description for Aquarius says they are.

Planet associated with: Neptune
Element: water
Comments: Water is also aligned with Neptune, both the planet and the god.  Michiru is a Pisces since her birthday is March 6th.  She also fits the personality type for Pisces, being sensitive, compassionate, artistic, and a pacifist (sorta).