Precession of the Equinoxes.

I’ve been thinking a bit about precession of the Equinoxes. Since the Earth is slightly oblate due to rotation and the axis is tilted relative to the plane of the elliptic, solar tidal forces exerts a “torque” on it that causes it to precess, like the wobbling of a top that’s a bit off center. It works out that the “wobble” takes 25,772 years to make one complete circuit. Now, as it wobbles, the dates on which the equinoxes and solstices occur will change. As it stands now, the Northern Hemisphere is angled toward the sun in June and away from it in December (why we have Summer and Winter. In 12886 years, the Northern Hemisphere will be angled toward the sun in December and away from it in June, giving us winter in June and Summer in December.

The year, per the Gregorian Calendar is 365.2425 days long. Divide the period of precession with the length of the year and you have that the equinoxes/solstices will shift 1 day for every 70.56 years. Now, I’m not clear on which direction it changes but I wonder, if it changes toward “earlier” then, it’s been 438 years since the Gregorian Calendar was first created. That’s enough to shift the Winter Solstice 6.2 days. This leads me to wonder if perhaps Christmas was originally intended to occur on the Solstice and they missed by couple days (early) and the current date of the Solstice is the result of precession of the Equinoxes.

Edit: On reflection, I think the Gregorian calendar is synced to the solstices (although unlike some historic calendars, they don’t start the year on the Solstice, the length of the year is based on solstice to solstice, rather than the Earth’s position in its orbit around the sun per se). Which would explain why 200 years ago it was still December 21st. The switch from the solstice being on December 10 (give or take a day) to December 21st (ditto) was in 1752, the same year the Gregorian calendar was adopted in the colonies that would become the United States.

Why, yes, as a matter of fact, I am a geek.

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