By Andrew Fraknoi and the Calendar in the Sky Team
Another suggestion you sometimes hear for how doomsday will arrive on December 21st, 2012 is that the Earth’s poles will experience a drastic shift, leading to disaster. Our planet has two kinds of poles – one pair from our magnetism and another one from the way we spin.
Our Magnetic Poles
The magnetic field of Earth is similar to that of a bar magnet. The magnetic poles are not aligned with the rotation (geographic) poles of Earth. The location of the magnetic poles changes with time and even flips with respect to the rotation poles over geologic history. In the current era, the magnetic pole in the northern hemisphere is a south magnetic pole, which is why the north pole of compass needles point in that direction. Image Credit: UC Berkeley
Because it has molten metals deep inside, conducting electricity, the spinning Earth acts like a giant magnet. It produces a magnetic field – a zone of magnetic influence that makes compasses turn. The field extends into space, shielding our planet from fast moving charged particles from the Sun. Just like electricity comes in two flavors, positive and negative, so magnetic fields have two directions to them, which we call north and south.
The two places where our planet’s magnetic field emerges from the Earth are called our north and south magnetic poles and they are not fixed. Over the time that careful measurements have been made, during the last two centuries, the north magnetic pole has shifted by hundreds of kilometers with no ill effects on our planet. This is because the electrical currents inside the Earth that are responsible for our magnetic field are quite complex, and can undergo many changes.
A timeline of magnetic reversals on Earth over the past 200 million years. Blue represents the same as our current polarity and red represents reversed polarity. Image Credit: UC Berkeley
Magnetic Pole Shift
In addition to the regular slow movement of our magnetic poles, from time to time the poles of our planet’s magnetic field actually reverse – the north becomes south and the south becomes north. Because the Earth’s magnetism is recorded in rocks that have metals in them (and can be magnetized), we have a good record of how the Earth’s magnetic poles have “flipped” over geologic times. The change of north to south (and vice versa) is not regular – tens of thousands to hundreds of millions of years can pass between field reversals.
We have not had such a change since human history began and we have no way of knowing when the next reversal will take place. What we do know is that the field reversal is not sudden, but takes place over thousands of years. Our entire planet’s field could not flip during a single day in 2012.
From the sheer number of magnetic field reversals in the geologic record, we can also deduce that life has survived the flip and continues to thrive and evolve on our planet. Geologists and space scientists who have studied such reversals tell us that the Earth’s field probably does not become zero during these times and that life may well continue to have some protection from the high-energy particles from space. Indeed, scientists who study the fossil record find no connection between the times that life-forms on Earth tended to die out in greater numbers and the times our magnetic field flipped.
In any case, we will likely have many, many years of warning when the next reversal takes place. In 2012, the magnetic shield that surrounds our planet is intact and not a source of concern.
Our Rotational Poles
Each planet orbiting the Sun – including the Earth – spins around an imaginary line (going through the planet’s center) which we call its axis. Where the axis sticks out of the planet is called its rotational pole. At the Earth’s north and south poles, seasons are the most extreme and the stars circle a point in the sky right above each pole.
The angle between Earth’s axis and the plane of its orbit around the Sun is tilted from the vertical by about 23.5 degrees and it wobbles in a gentle circle every 26,000 years or so. Although this seems like a long period compared to human lives, we can measure it pretty accurately these days. There is also an even slower (and smaller) change in the angle by which we are tipped, going from 22 degrees to 24.5 degrees over a period of 41,000 years. This slower motion is also something we can now measure with a great deal of accuracy.
Not every planet has a tilt to its axis – Venus’ axis is barely tilted, while Mars’ is tilted a bit more than ours. Why do some planets have a tilted axis? When the Solar System was first forming, billions of years ago, it was a much more violent place than it is today. There were many more mini-planets orbiting the Sun on a variety of different paths. Occasionally, some of the forming planets came too close for comfort and collided. Overwhelmed by a large enough chunk of material (with its strong gravity), the axis of a young planet might tilt one way or another. It probably took several giant collisions or near-collisions to get the Earth into its current state of leaning.
Earth's rotation axis is an imaginary line that Earth spins around. The axis passes through the center of Earth and the geographic poles of the planet. The poles are the places that do not move but turn in place as Earth spins. Earth's axis is tilted relative to its orbit around the Sun (called the ecliptic). Image Credit: Dennis Nilsson
Rotational Pole Shift
Today’s Solar System is a well ordered place. Each of the eight planets moves in a nearly circular orbit around the Sun. The only debris left is the occasional comet (piece of ice) or asteroid (piece of rock) moving between the planets. Nothing large enough to change a planet’s tilt is left over from those early days.
Yet, rumors continue to circulate that some cosmic event or process is either going to change the Earth’s tilt drastically in December, or will actually turn our planet upside down – with the north pole winding up where the south pole is. The energy required to take our entire planet and tilt or flip it in this way is so large it boggles the mind. If something massive enough to do this were coming our way (and a planet the size of Mars or bigger on a collision course with Earth would be needed), astronomers would long ago have noticed it. Its gravity would have disturbed the orbits of the other planets as it was passing. And the sunlight reflected from the intruder would have made it one of the brightest objects in our sky.
There is simply no evidence of any such interplanetary intruder, and no other source of energy to change a fundamental motion of the Earth is in sight. One possible source for these rumors could be confusion between the Earth’s rotational and magnetic poles. If you don’t know the difference and you hear that the Earth’s poles can undergo a flip, you might think that scientists are talking about the rotational poles and not the magnetic poles. But they are an entirely different phenomena and neither is a reason to stop planning for a happy and productive 2013.