These are the very last days of July, 2013. If we don’t stop and capture the sunshine and the gentle trade winds, the scented leaves and the birdsong, it will be over in a matter of hours and we will be neck-deep in the warm beach sands of August. But that is true only until it is no longer true. In fact, the clock is ticking towards that day when the warmth of July will be a thing of the past.
In fact, if we fast forward 11,000 years, it will be winter in July. And it won’t happen because of anything anyone has ever done or will ever do. It will happen because of something called General Precession.
Ah, the sound of sleigh bells jingling in the bright cold air of a summer that has come once again to the denizens of a beautiful Earth, celebrating yet another summer vacation indoors while the snow plows work mercilessly, clearing away the streets so that folks can go out and visit with friends while enjoying their favorite things, such as crisp apple strudels and warm woolen mittens.
So what is this general precession thing that causes winter to occur in July? Well, it’s not really a thing but rather, more like an action. Wikipedia describes it as the change in the orientation of the rotational axis of a rotational body (the Earth) in which the second Euloer angle is constant. ~ Wikipedia 
Now, I don’t know about you, but that’s just nonsensical blathering to me. However, since it is rather important to me to understand why my favorite time of the year is going to eventually become cold and unappealing, I am going to try to figure this out using my less-than-stellar Taobabe brains. So let’s break it down into its components.
This is the WHAT and the HOW:
So what the heck is a Euloer angle? How do you even pronounce that? Is it someone who delivers eulogies? No? I didn’t think it was that simple—but in this case, it really is simple. Notice that this spinning top has one point (at the bottom) that does not move. It’s dead stuck in one spot. This is the Euloer angle, and it is constant because it never moves. Now, take a look at its opposite end (at the top). That one is moving all over the place. The movement is so wide that it causes a serious wobble to the sphere. That is what is meant by a change in the orientation of the rotational axis.
Now let’s get into the WHY:
If it was just the Earth, spinning by its lonesome, with only its gravitational weight to make it bulge here and there (see my previous post Lumpy Bumpy Humpty Dumpty), the effect of its planetary precession would be so small, we would barely notice it. In truth, the Earth is part of a whole system of other bodies, weighing in and contributing to that gravitational pull, and this pull is ridiculously strong.
Consider that our very own Moon (aka Luna), is just about one of the biggest moons to be found in our solar system, and then consider that Earth is a rather small potato when compared to all the other giants orbiting the Sun, and we have a strange anomalous relationship between a rather small planet and a rather large satellite. That lunar pull is so strong that it actually causes Earth to bulge out around its midsection. Add the gravitational pull of the mighty Sun to this equation and the Earth’s equatorial region is completely squashed out of shape.
With both the Moon’s and the Sun’s combined gravities exerting their influence on the Earth, they collectively yank her chains, so to speak, and Earth’s spin becomes noticeably wobbled and teetered. The action of Sun and Moon on poor Earth is what is known as the lunisolar precession and it is about 500 times greater than planetary precession alone. And before you even have to ask, yes all the other planets exert their pull as well, but their influence is slight compared to the lunisolar precession’s pull so I have decided to leave them out of the mathematical equation.
Mathematical equation? Did I just say that dreaded ‘M’ word? Why, yes—yes I did, but only because I don’t have to do the hard work of calculating this horrid thing out to its bitter end. I am simply going to let smarter people do the math and then point to it and say, “See? That’s how it works.”
- Gm = standard gravitational parameter of the perturbing body
- r = geocentric distance to the perturbing body
- C = moment of inertia around Earth’s axis of rotation
- A = moment of inertia around any equatorial diameter of Earth
- C − A = moment of inertia of Earth’s equatorial bulge (C > A)
- δ = declination of the perturbing body (north or south of equator)
- α = right ascension of the perturbing body (east from vernal equinox).
No need to look at the equation closely. I swear, the numbers work out correctly.  (I stole it from Wikipedia so it must be correct. N’est ce pas?). Anyhoo, onto bigger and better things.
This is the WHEN:
The Earth’s precession is very, very, slow. How slow? From start to finish, it is a 25,700 year cycle. We have already passed the half-way point of this latest precession cycle, which happened two-thousand years ago (give or take a few years). In other words, right around the time Jesus was born, we hit mid-cycle.
We are now moving into the second phase of the cycle, which means our summers will eventually get colder and colder until it becomes winter in July, 11,000 years from now.
Yes, this will happen.
No, this has nothing to do with CO2 or human-created global warming. It does not matter what we do or don’t do, and it doesn’t matter if we’re even around to watch it happen. It will simply happen when the time comes for it to happen because it is a natural and rhythmic cycle which exists outside of and beyond our sphere of influence. It has been in existence since the world began and it will not end until Earth is subsumed by the Sun (or is otherwise destroyed due to some cataclysmic event beyond our imagination).
Of course, we’ll all be safely dead by then, so this won’t really have much of an impact on us—-except for a couple of OTHER things about Earth’s movement, which I am a bit more concerned with. You see, general precession is just one of three Earth cycles that a gentleman by the name of Milutin Milankovitch has so very kindly mapped out for us back in 1920.  Even though we won’t be around to witness the next precessional winter in July, the other two cycles will be interesting to live through, as they are coming ’round the bend in short order.
(…to be continued)