Will Earth's tilt angle continue to increase?
An illustration of the effects of Earth's precession. Image from NASA's Goddard Space Flight Center.
The Earth’s axis is now tilted at about twenty-three and one-half degrees. Over a long time, will this tilt angle increase so that the Earth’s wobble will become ever ‘flatter’ like that of a spinning top?
Maybe you’ve noticed that a top will start to wobble after it spins for a while. This wobbly motion is called “precession.” As the top loses energy through friction, it tilts more and more – until it falls over.
Like a top, our Earth spins on its axis and over time it too precesses. One completion of Earth’s precession cycle takes about 26,000 years. Precession is mainly caused by the tug of the moon and the sun on Earth’s equatorial bulge. But the moon and sun also cause the rise and fall of ocean tides. Tidal forces act to slow down Earth’s spinning motion. Will Earth’s axis begin to wobble more and more? The answer is that no one knows for sure.
If the moon’s gravity had its way, it would force Earth’s axis tostraighten up and be at right angles to the moon’s orbit. But the sun exerts its pull on Earth, too – so what we really have is a three-way tug-of-war. Even the experts can’t predict exactly what will happen to this system over billions of years.
When thinking of Earth’s procession, here’s more to think about:
1. If you drop a spinning top, it will freely fall in the
gravitational field, maintaining its spin direction. A top mounted in
gimbals (so there are no unbalanced forces applied) always points in the same direction. This feature is used to make gyro compasses.
2. The top precesses rather than falling because of the vector nature of angular momentum. The intrinsic angular momentum vector of the top points along the axis. The turning couple provided by gravity and the reaction force from the table tries to tip the top, but this change in angular momentum is at right angles to the spin and parallel to the table surface, so the tilt direction does not fall—it moves sideways. This happens continually, so the top precesses.
3. The motion of a spinning top can get even more complicated: the tilt angle can oscillate around an average value as the top precesses. This motion is called nutation.
4. The precession of the Earth’s axis causes the apparent position of the celestial north pole to move among the stars in the sky. In
13,000 years, the nearest bright star to the pole will be Vega, not
Polaris.
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