As Mamoru learned, never forget your girlfriend’s birthday (ep. 101)

I don’t think I need to really go out and say that dates are a little strange in the Sailor Moon universe, especially when it comes to calendars, dates, and ages. The series, in the manga at least, does a surprisingly good job compared to other long-running series, but that’s a story for another time. But one thing that always stood out to me is: why, exactly, do ChibiUsa and Usagi have the same birthday?

On the surface, it seems like it’s mostly a device for Ms. Takeuchi to continue with the theme of Princess Usagi Small Lady Serenity and her mother being so similar – down to the name, odango/hair decorations, and personality. In fact, when you look back at the original reference material, you’ll find that Usagi herself was to have pink hair,¹ so in its own way, even the points where she differs are an homage to her mother, Usagi.

It turns out, though, that Ms. Takeuchi was very careful in how she picked out birthdays for the characters in the series. Just like she had with blood types – a common form of personality analysis and fortune-telling in Japan² – all of the birthdates for the Sailor Soldiers were chosen by a mix of their personality, element, and guardian planet. Considering that this would put Usagi and ChibiUsa under the same planet,³ Ms. Takeuchi was basically backed into a corner with a one month window when it came to candidates for ChibiUsa’s birthdate.

Pretty Soldier Sailor Moon Sailor Stars – Zodiac Chart

So, then, how was Usagi’s birthday decided in the first place? According to Ms. Takeuchi, Usagi was born on June 30, which would make her a Cancer according to the zodiac calendar.⁴ While we’re pretty hard-pressed to say that Usagi has a lot to do with crabs or the element of water, it turns out that the ruling celestial body of Cancer is the Moon. To summarize really roughly,⁵ the ruling celestial body, or the domicile therefor, for a zodiac sign is the body which is considered to have the strongest influence over the sign (and, by extension, the people born under it). Only seven celestial bodies – two lights, the Sun and the Moon, and five planets (literally “wanderers”)⁶ at the time – were known to ancient astronomers at the time, so though some bodies were given two star signs, the Sun and the Moon were granted the summer months due to being the warmest in the northern hemisphere.

This rulership / domicile system actually plays out for all the remaining Sailor Soldiers as well, even down to Mamoru. Born on August 3rd, that actually puts him under the star sign Leo and under the rulership of the Sun, but as mentioned before, the Earth was still considered to be the center of the universe, leaving the Sun as the closest alternative. This also also nicely contrasts with Usagi’s Moon, and avoids conflict with other Sailor Soldiers, so that makes it a pretty natural choice for Mamoru.

Happy Birthday to Usagi and Usagi!

In short, the reason for ChibiUsa and Usagi having the same birthdate was likely two-fold: it worked nicely with the play on ChibiUsa being (as her nickname implies) a copy of Usagi and, ultimately, the only real choice for her birthday was within one month from Usagi’s if Ms. Takeuchi wished to stick to her established system, so having the same birthday really made the most sense.

In case anyone’s curious or wants to see how the rest of the birthdates play out, the following is a table of all of the Sailor Soldiers, their birthdates, relevant zodiac information, and blood types.

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