Happy Leap Day!
Happy Leap Day, everyone. February 29 is just such an odd thing. I blame Pope Gregory XIII, because clearly this whole “leap year” thing is all a Vatican plot to keep Easter in March or April.
OK, fine. It might not actually be a “Vatican plot” — though that did get your attention, haha — but the current implementation of “leap year” does, in fact, keep the vernal equinox (and therefore Easter) close to March 20 in the Northern Hemisphere. It takes the Earth more than 365 days to make one complete circuit around the sun; depending on how you calculate a full trip (HUGE can o’worms there), it really takes around 365 days 6 hours. To keep the civil calendar somewhat aligned with the usual astronomical markers, the Romans tacked an extra day onto the end of February every four years. Without leap day, the civil calendar would be slow relative to the astronomical year, and the northern vernal equinox would creep backwards into February, January, and so on.
Leap year is also one reason why the astrological signs we generally think of in the West no longer align with the constellations for which they’re named. Good planetarium-style software (I’m fond of Stellarium) can show this effect; at the 2008 northern vernal equinox on March 20, the sun is actually in the constellation Pisces, but the “tropical” zodiac (which most Westerners know best) defines that equinox as the beginning of the sign Aries. Without leap year — or in the “sidereal” zodiac — the different signs drift among the seasons, with spring coming in Aries, then Pisces, then Aquarius, then Capricorn, etc. (For the record, the concept of the “Age of Aquarius” is related to the position of the Sun relative to the sidereal zodiac at the time of the March equinox; when this position shifts from Pisces to Aquarius, then “peace will rule the planet and love will steer the stars” as the song goes.)
Lots more can be said here, but I’m getting too deep in Wikipedia articles for my own good. In any case, enjoy the rest of your intercalation.
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