Talk:Precession

The sentence in section "Precession of the equinoxes" reading

The brightest star to be North Star at any time in the foreseeable past or future is the brilliant Vega, which will be the pole star in 14000 AD.

fails to reflect the fact that "foreseeing" inherantly means future.

I think the concept being groped for here is that there are may be stars hidden from view and thus not subject to our extrapolations brighter than the pole stars we can extrapolate forward or back to; i don't know whether increasing errors with increasing time interval relative to the present become an issue. Setting forth actual numbers for the time range intended would be informative and more credible.

But i'm making a hack at a temporary improvement. --Jerzy(t) 04:28, 2004 Mar 27 (UTC)

Physics - first sentence

Precession is due to the fact that the resultant of the angular velocity of rotation and the angular velocity produced by the torque is an angular velocity about a line which makes an angle with the permanent rotation axis, and this angle lies in a plane at right angles to the plane of the couple producing the torque.

This sentence is quite confusing to me (and I believe many others). Could someone try to clarify it a bit? After reading it over several times and trying to think it through, I still don't believe that I have an adequate understanding to reword it myself.

I'd like to also make a nice diagram (like the one I made for Vortex ring), but first I've got to figure out the actual physical concept... ✈ James C. 05:48, 2004 Aug 22 (UTC)

Physics - first sentence revisited

Precession is an angular velocity produced by the resultant of the angular velocity of the rotating mass and the angular velocity generated by the torque. The axis of Precession lies at right angles to the permanent rotation axis (axis of the rotating mass) and the rotation axis resulting from the torque.

That is how I understand Precession. Is that correct?

The equation is also confusing me. Let me explain with an example; in a video at http://science.howstuffworks.com/gyroscope1.htm , I saw a bicycle wheel tied via one end of its axle to a string which was itself tied to the ceiling. Some person was holding the bicyle wheel such that the axle was horizontal (bicycle axle and string make a 90 degree angle), proceeds to spin the tire and let go.

Had the wheel not been spinning or spinning very slowly, intuition says the bicyle wheel would rotate such that the axle and string become parallel. However, because the bicyle wheel was rotating itself, instead of rotating such that axle and string align, the bicycle wheel instead begins to rotate about the string, axle remaining horizontal.

Back to the equation:

<math>

T_p = \frac{4\pi^2I_s}{QT_s} <math>

In which I_s is the moment of inertia, T_s is the period of spin about the spin axis, and Q is the torque.

T_s is a measure of time defining a set rotation, say 360 degrees. Q is the Torque created by gravity pulling on the mass of the tire (approx mid axle) against the string (tied at one end of the axle).

Now, because of where T_s is located in the equation, keeping all other variables constant, increasing the duration of one 360 degree rotation decreases the duration of the precession spin. IE the precession spins faster about the string as the bicycle tire spins slower about its axle. That just doesn't make sense.

Where am I going wrong?

One way of interpreting that situation is to bear in mind that the faster the rate of rotation, the larger the angular momentum. The larger the angular momentum the smaller the response to a torque that is applied. If I hazard a guess I'd say that if the torque is increased proportional to the increase in angular momentum then the rate of precession remains the same. --Cleon Teunissen | Talk 22:50, 10 Mar 2005 (UTC)

Thought experiment to help visualisation of the dynamics.

I am thinking about inserting the following section I wrote in the article, or replacing a section. Any comments are welcome. --Cleon Teunissen | Talk 22:38, 10 Mar 2005 (UTC)

Imagine a perfectly spherical spinning asteroid. A monorail track is build, in a perfect circle all around the asteroid. Initially the monorail track goes over the geographic north pole and south pole of the spinning asteroid, but it can slide, remaining a flat ring as the whole of the track slides. (The direction of the spin determines the geographic poles and the equator). Positioned on the monorail is a continuous train of wagons, all around the asteroid. Initially the monorail track and the train on it are co-rotating with the asteroid.

Next, imagine that on all wagons small rocket engines are started, exerting thrust. The combined action of those thrusters is a torque around an axis that passes through the equator. Then to visualize what will happen it helps to divide the picture into four quadrants. 1) a section of train that is moving from the north pole to the equator, 2) a section moving from the equator to the south pole, 3) a section moving from the south pole to the equator, 4) a section moving from the equator to the north pole.

Then in the northern hemisphere the coriolis effect will shift the track in one directon, and on the southern hemisphere in the other. The combined action of the coriolis effect in the northern and southern hemisphere is that there is a torque around an axis that passes through the equator, and that is perpendicular to the axis of the torque of the thrusters. The monorail track will slide over the asteroid, and eventually it will come to rest parallel to the equator.

In this visualisation only a part of the rotating system undergoes precession. Normally precession refers to a solid, spinning body on which a torque is applied. The 'coriolis effect' usually refers to the motion of a small mass relative to a much heavier rotating mass. This visualisation shows that precession and coriolis effect are related phenomena. Precession is manifestation of the coriolis effect in the case of a single solid body on which a torque is applied.

--Cleon Teunissen | Talk 22:38, 10 Mar 2005 (UTC)