I never see the term precession without thinking of “How the Rhinoceros Got His Skin,” one of Kipling’s Just So Stories — because it starts right out on an island that is located near “the promontories of the larger equinox,” an entirely absurd description, but an echo of the phrase “precession of the equinoxes” and so like the way that small children experiment with big words that sound nice and juicy to them.
The equinox, of course, has no promontories, merely being the day of the year that night and day are the same length for everyone. Nevertheless, the stars that are highest in the sky at the time of the equinox change over hundreds of years, and because this change is due to a wobble called precession, in the movement of Earth’s axis, it is called the precession of the equinoxes. To be more precise, precession is the change in the axis of the earth so that it does not always point north to our good friend Polaris, but, after a while, to other stars in the Little Dipper, even to Vega and on to other stars before returning to Polaris again.
Imagine a spinning top. It may begin with a spin so perfect it seems to be standing perfectly still. Then it may begin to lean, and when it leans, the leaning axis slowly revolves, pointing slightly east, then north, west, and south and back to the east in succession. All these changes may or may not include a change in how much it leans (may or may not include a change in its obliquity). Just the change in direction, however, means a change in where the axis points, and if stars were painted on the ceiling, you would see various stars take turns being the “north star” to which the axis points.
So how does that change the weather? Should the Polaris turn around and blow snow upon us when we wander away?
I think not.
But something else happens.
As you know, the winter of the Northern hemisphere comes when Earth is nearest the sun, and the winter of the Southern hemisphere comes when it is farthest. That could be expected to make the southern winters much more intense than the northern ones, because the cold of being tilted away from the sun is added to the cold of being far away from the sun. Yet it does not happen so.
If you look at a globe, you will see why. The southern hemisphere doesn’t have as much land as the northern hemisphere, and this changes how heat moves in the south. Vast ocean currents – rivers within the ocean — carry heat and cold all around the globe, and these moderate the potential extremities of the Southern winter.
Now turn it around.
What happens when the northern, land-rich hemisphere is turned away from the sun during its winter? This happens when the axis points to Vega for its north star. In this case, the cold of being far away is added to the cold of being turned away, and there is no world-circling current of water to moderate it. That could be pretty intense, right? And even though there are world-wide winds which mix around and some of them (from the northern and southern Hadley cells) meet regularly at the equator, they don’t mix that well, and even if they did, air doesn’t change the land heat near as readily as the ocean mixes itself. Remember all those things about phase change in water? So those northern lands are much more apt to freeze than water than water!
Indeed, when the northern hemisphere is both far from and pointed away from the sun, its winters are much tougher, and the whole planet cools. So, again, here is a cycle that takes a very long time – 23,000 years – and it has a strong effect on climate.
Actually, it should take 26,000 years, but the whole orbit of Earth is turning so that the precession gets carried along and saves 3,000 years.
Did that last bit leave you a little confused? Join the club. It takes a long time to integrate so many motions into a single, flowing image. Left to yourself, you’d probably never come up with it, would you?
So you might not be a civil engineer, such as Milutin Milankoviç!
(Or you might be one someday, but not yet.)
These three long-term shapers of weather – the eccentricity of the earth’s orbit, and the obliquity and the precession of its axis — are called the Milankoviç cycles. People argue about them; some say they don’t matter. They do matter! They do shape weather! Or rather, they shape that larger thing, climate, in which the little blips called weather take place. But they are so long-term, and they get piled on top of each other in so many ways that only a serious mathematician would even dream of sorting them out. We’ll take a look some time.
Meantime, you learn everything you can about wind and sun, water and pressure, dryness and precipitation, and then there is something immensely important that you didn’t even consider.
There’s always more.
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