# Two types of precession?

• jdstokes
In summary, there are two main forms of precession: one where a torque is induced on a spinning object causing it to rotate about a vertical axis, and another where the direction of induced rotation is different from the direction of the induced torque. The second form can be seen in the experiment with a spinning bicycle wheel supported by a rope. There are also two other forms of precession: orbital precession, seen in the changing semimajor axis of orbiting planets, and precession of an object's axis of rotation when it is not aligned with a principle axis. This type of precession can be observed in a free-falling, rotating box.

#### jdstokes

It occurred to me recently that there are two forms of precession.

If you hold a spinning bicycle wheel and force the axis of rotation to tilt on its side, this will induce a torque tending to spin the wheel about a vertical axis. This is in accord with the fact that the change in angular momentum is in a vertical direction.

On the other hand, in the experiment where a spinning bicycle wheel is supported on one end of the axis by a rope and undergoes precession, the direction of the induced rotation is not the same as the direction of the induced torque (which changes over time).

I have always understood the second experiment as an example of precession. Would one put the first example into this same category?

I can think of two other forms of precession although you can't exemplify them with the spinning bicycle wheel.

There is orbital precession where the semimajor axis of an orbiting planet changes over time. The deviation of Mercury's orbital precession from Newtonian predictions helped confirm Einstein's GR.

The other occurs when an object is freely spinning with no torques but the axis of rotation is not through a principle axis. Imagine a rectangular box in freefall (initially) rotating about the diagonal through opposite corners. What occurs is that the axis of rotation is not parallel to the direction of the angular momentum. The angular momentum is conserved but the axis of rotation will precess about the direction of the angular momentum. What is more the axis of rotation relative to the object will oscillate between principle axes.

## 1. What is the difference between axial and orbital precession?

Axial precession refers to the slow, continuous change in the direction of Earth's rotational axis, while orbital precession refers to the gradual change in the orientation of Earth's elliptical orbit around the sun.

## 2. What causes axial and orbital precession?

Axial precession is primarily caused by the gravitational pull of the moon and sun on Earth's equatorial bulge, while orbital precession is caused by the gravitational pull of other planets in our solar system.

## 3. How long does it take for axial and orbital precession to occur?

Axial precession has a cycle of approximately 26,000 years, while orbital precession has a cycle of about 112,000 years.

## 4. What effects do axial and orbital precession have on Earth?

Axial precession can affect the length of seasons and the position of Earth's poles, while orbital precession can impact the strength of Earth's tides and the timing of solstices and equinoxes.

## 5. Are there other types of precession besides axial and orbital?

Yes, there are other types of precession, such as nutation (small oscillations in Earth's rotational axis) and gyroscopic precession (the change in orientation of a spinning object due to an external force).