# Precession axis of a spinning top

1. Sep 23, 2006

### TopcatBV

Hi, I have a little question

The precession axis of a spinning top is represented vertical in every pages I have seen. Why is that?
Isn't that axis just a virtual (resulting from spinning of the precession axis) precession axis, because the precession axis must be perpendicular to momentum and the spinning axis of the top?
I attach a picture from Feynman's lecture (pa1) and one from me (pa2).
Thanks

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2. Sep 26, 2006

### TopcatBV

The formula τ=ΔL/Δt=Δθ/ΔtxL=ΩxL mislead me to think that Ω is perpendicular on L, but it is not! So Ω is in vertical direction because Δθ is horizontal because τ is horizontal because F is vertical and my picture is wrong

3. Apr 12, 2007

### Abel Cavaşi

Ok, but the Earth have the precession axis perpendicularly on the ecliptic! Is that correct?
I am convinced that no! If the Earth was an spinning top in the gravitational field of the Sun, then the precession axis of the Earth must be parallel with the gravitational forces of the Sun!

4. Apr 20, 2007

### lpfr

The precession of earth, called precession of the equinoxes is due to the flattening of the poles of the earth.
If the earth where spherical the force exerted by the Sun will do as if it was exerted on the center of the sphere. But it is not the case. The gravitational force is inversely proportional to distance.
Now imagine the earth in summer, North Pole inclined to the Sun. The bulge of the earth opposite to the Sun is attracted a little less than the bulge in the side if the Sun. An these forces are not in the same plane. One is in the north side of the ecliptic and the other in the south side. As the forces are not identical there is a torque. This torque is parallel to the orbit of the earth and, of course, to the ecliptic.
The variation of angular moment is parallel to torque and then parallel to the ecliptic. That is, the axis of precession is perpendicular to the ecliptic.
You can see
http://en.wikipedia.org/wiki/Precession_of_the_equinoxes
for good diagrams and illustrations and a little longer explanation.

Last edited: Apr 20, 2007
5. Apr 22, 2007

### Abel Cavaşi

Last edited by a moderator: Apr 22, 2017
6. Apr 22, 2007

### lpfr

Last edited by a moderator: Apr 22, 2017
7. Apr 22, 2007

### Abel Cavaşi

Oh, so you agree that the explanation from Wikipedia is wrong?

8. Apr 22, 2007

### lpfr

I make a mistake: I forgot the vertical components (they are NOT negligeable).

The explanations that Tauʻolunga gave you are the good ones.

9. Apr 22, 2007

### Abel Cavaşi

The vertical components doesn't exist because the forces are coplanare.

10. Apr 22, 2007

### lpfr

NO. Your are wrong. Do the correct computation. The Sun is far but not at infinity.

11. Apr 22, 2007

### Abel Cavaşi

I didn't say that the forces are parallel, but coplanare!

12. Nov 15, 2009

### Polestar101

Abel - I looked at your wikipedia page. Your dynamics are logical and your math is correct. Few people realize that Newton's precession calculations did not work. It was only after d'Alembert and others began to greatly modify the assumptions about the rigid earth that the values were forced to fit the observable. But even this approach has failed to predict "changes" in the precession rate (example, the rate has been steadily increasing even though it should be decreasing because the moon [largest force] is slightly receding from the earth and the earth slightly receding from the sun [second largest force].

Something is fishy with the model. In 2006 the International Astronomical Union determined the precession-nutation model was "inconsistent with dynamical theory" (IAU P03). I would not be surprised to learn someday that precession has a completely different cause than nutation. This is a fun problem! Write me at <email address deleted> and let's compare notes.

Last edited by a moderator: Nov 16, 2009
13. Nov 16, 2009

### Abel Cavaşi

Hello, Polestar101! Your observation is very relevant on that theoretical predictions of the rate of precession does not coincide with observations. I hope from my heart to give them much to think those who still believe the current explanation of the precession of the Earth.

I am overjoyed that you want to deepen this subject of great future, but I think it would be better to talk in public about what we know because we have nothing to hide. Aren't?

14. Nov 16, 2009

### D H

Staff Emeritus
To Abel Cavaşi and Polestar101: Both of you are in violation of this site's rules.

To everyone else: These two are crackpots.

Abel Cavaşi does not understand how precession arises. The Earth obviously is not a spinning top on some surface. Some model other than a spinning top is needed to explain how planetary precession occurs. The correct model is to use gravity gradient torque.

Polestar101 is quoting the IAU out of context. Several organizations, including the IAU, the IERS, NASA, and the US Naval Observatory, are involved in a never-ending effort to improve the reference frames used in the sciences. Two of the most important (to us) reference frames are inertial and Earth-fixed reference frames. Accurately knowing and predicting the relationship between these frames is the subject of precession/nutation theory.

Note very well: The error in Earth orientation is now known to within 300 micro arcseconds, RMS, with an error growth rate of about 30 micro arcseconds per year. (About those numbers: For a circle the size of the Earth, the arc length subtended by a 300 micro arcsecond angle from the center of the Earth represents about 1 centimeter at the Earth's surface.)

Note very very well: No new science was needed to create a model of this precision. Scientists have a very good model of what causes precession.

15. Nov 16, 2009