Talk:Precession

message: 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)
Great article, though I have little idea about physics. I was coping until the example was given around torsional precession talking about gimbles and the like. Perhaps better labelling of the components of motion so that there is no ambiguity, and a sequence of diagrams illustrating the points may be helpful. thanks adam  —Preceding unsigned comment added by 165.228.172.109 (talk) 00:52, 24 December 2009 (UTC)
thread_title: Discussion

message: :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:
:
T_p = \frac{4\pi^2I_s}{QT_s}
In which Is is the moment of inertia, Ts is the period of spin about the spin axis, and Q is the torque.
Ts 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 Ts 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)
thread_title:  Physics - first sentence revisited 

message: :how do newtonian dynamics account for the fact that the equinox precesses relative to objects outside the solar system, but not for objects inside the solar system? Ungtss 17:06, 16 August 2005 (UTC)
replies: text: thank you for taking time with me:).  let me try to articulate my question a little better.  the lunisolar theory holds that precession is caused by a "wobble" in the earth's rotational axis.  it seems to me that if this were true, the wobble in the axis would cause everything in the sky to appear to move, including the sun.  but our observations show that the location of the sun and the outside stars change relative to each other.  the same thing with the planets.  to my knowledge, while calculations of the location of the stars take precession into account, calculations of the location of the sun and planets do not.  on the contrary, while the equinox appears to precess relative to the stars, it does not appear to precess relative to the sun or planets.  this, within the binary model, implies that the entire solar system is moving in a curved path.  here is a link -- i'm far from an expert and i would greatly appreciate your thoughts on this.    Ungtss 19:11, 16 August 2005 (UTC)
thread_title:  Gyroscopic precession relative to what? 

message: I think it should be emphasized that 'precession of the Equinoxes' is a misnomer. It suggests a connection with motion of the Sun that isn't there. The Gyroscopic precession of the Earth is unrelated to relative motion of the Sun and Earth.
As seen from the perspective of the Earth the Sun takes a year to go around the Earth's orbital plane (the ecliptica).<BR>
Two times a year the Sun happens to be on the line of intersection of the Earth's equatorial plane and the ecliptica.
The gyroscopic precession is measured by counting how long it takes for the line of intersection of the Earth's equatorial plane and the ecliptica to complete a full 360 turn. That takes about 26.000 years. It's that line of intersection that counts. --Cleon Teunissen | Talk 19:35, 16 August 2005 (UTC)
thread_title:  Gyroscopic precession of the Earth is unrelated to relative motion of Sun and Earth 

message: Recapitulating:<BR>
The solar day is the amount of time between two consecutive sightings of the Sun by an observatorium, at exactly the same angle. For example the Sun passing straight overhead over the Greenwich meridian, and the next pasage the next day.
The siderial day is the amount of time it takes for the Earth to complete a full rotation, a sidereal day is 23 hours 56 minututes, 4 second.
In for example meteorological models the rotation rate of the Earth is taken into account. A meteorologist told me he had at one time, out of curiosity, tried what would happen if he inserted the wrong rotation rate into the model. Sure enough, when he used the solar day, the quality of the projections was less good than with the correct rotation rate.
Something similar is the case with the duration of the year. The sidereal year is the true year. The tropical year is an artifact of circumstances, and in that sense it is not particularly interesting from a physics point of view. --Cleon Teunissen | Talk 22:42, 16 August 2005 (UTC)
replies: text: you've said that the "sidereal day/year" are the "real" day and year, and the tropical day/year are merely artifacts of circumstances.  the heart of the binary argument is that the tropical day/year are the real day/year, and the sidereal day/year are the artifacts of circumstances.  i understand that the mainstream view holds to the former, but i don't see how they can prove that, without disregarding a substantial amount of evidence regarding the fact that all objects within the solar system are predicted using the tropical year, while only objects outside the solar system are predicted by using the sideral year.  that fact appears to me to falsify your conclusions about which is the "true year."  Ungtss 16:26, 17 August 2005 (UTC)

text: as i understand it, the locations of objects within the solar system, including solar eclipses, are always calculated with respect to the tropical year.  is this not true?  Ungtss 16:26, 17 August 2005 (UTC)
thread_title:  Solar day and sidereal day 

message: Quote from site about planets and their moons
:Of all moons in the solar system, Nereid has the most elliptical orbit around its planet. When Nereid is at its closest to Neptune, its distance from the planet is only about 1.35 million km (about 812,000 mi). At the farthest point in its orbit around Neptune, Nereid is about 9.624 million km (about 5.77 million mi) from the planet. Nereid’s orbit is tilted 28° relative to Neptune’s equator. <BR>[end quote]
I expect the axis of the elliptical orbit of Nereid to keep pointing to the same fixed stars (With any deviation accounted for in terms of expected perturbation) 
The fact that Neptune is orbiting the Sun does not in itself influence the direction of the axis of the elliptical orbit of Nereid.
Likewise any gyroscopic precession of any of the solar system's moons. Those gyroscopic precessions are independent of their parent planet's orbit around the Sun. --Cleon Teunissen | Talk 08:38, 17 August 2005 (UTC)
replies: text: precisely.  the binary argument is that the moons and planets do not tilt relative to the objects around which they are revolving.  the argument is that they do not change their tilt to any substantial degree at all.  instead, the argument is that because of the revolution of neptune, the location of neptune in Nereid's sky on its "vernal equinox" with neptune will appear to move slightly relative to the background stars on every "year" of nereid's orbit, because of neptune's revolution around the sun.  they're saying there is no gyroscopic motion at all, but that the observed precession from the earth's surface is not a result of motion in the axis of the earth, but a result of curved motion by the sun.  is this any clearer?  Ungtss 16:48, 17 August 2005 (UTC)

text: the binary argument is that the moons and planets do not tilt relative to the objects around which they are revolving.  Ungtss 16:48, 17 August 2005 (UTC)
thread_title:  Neptune's moon Nereid 

message: I propose to remove the remark about torque-free precession. Or if it is mentioned at all, then much further down in the article, for it is a very minor detail. --Cleon Teunissen | Talk 22:08, 31 August 2005 (UTC)
thread_title:  Torque-free precession and torque-induced precession 

message: The article says: As a spinning object precesses, the tilt of its axis goes around in a circle in the opposite direction that the object is spinning. 
But it is not so for a toy top. I've just got one of these and started it rotating clockwise. I've clearly seen that the axis precessed clockwise too, apparently not counter-clockwise, as I expected from what I've read here. Only when it fell at last, it started moving counter-clockwise, due to friction with the surface.
:I would agree that precession of a toy top seems to be in the same direction as the spin of the top, in the sense that (looking from above) the top and the ends of the axis seem to rotate in the same direction.  When the top falls over the is usually fricitional slippage followed by a rolling rotation where the end of the axis seems to rotate in the opposite direction, but the top itself rotates in the original direction relative to the axis. --Henrygb 15:09, 8 February 2006 (UTC)
replies: text: I agree that the article is misleading. The direction of precession depends on the direction of rotation and the direction of the applied force. In the case of a spinning top, gravity is trying to make it fall over. In the case of the Earth, the gravitational forces of the Sun and Moon on the spheroidal shape are trying to right it – hence the north pole precesses in retrograde motion. --Portnadler 18:30, 8 February 2006 (UTC)
replies: text: I agree with Pordnadler; In the case of the Earth the gravitation of Sun and Moon exert a torque on the oblate spheroid that tends to align the Earth's equatorial plane with the plane of the Earth's orbit. (By the way, I find it pretty amazing that the gyroscopic precession has been preventing that alignment for as long as the Earth has been in existence.) A toy top that is suspended on string that is attached to the top end of the toy top's axis of rotation will precess like the Earth does. --Cleonis | Talk 19:10, 8 February 2006 (UTC)
replies: text: I've simply removed the inaccurate sentence. If anyone can come up with a better explanation, please feel free. --Portnadler 11:25, 12 February 2006 (UTC)
thread_title: Direction of precession of a toy top

message: Weisstein has an article called free precession, but it says that there is constant torque, rather than no torque. Is this incorrect, the same thing or something different than torque-free precession? -- Kjkolb 14:34, 26 November 2005 (UTC)
replies: text: -- Kjkolb 07:53, 28 November 2005 (UTC)
thread_title:  Other types of precession 

message: My bible for these matters is Simon Newcomb's A Compendium of Spherical Astronomy, The Macmillan Company, 1906, republished by Dover, 1960. This makes the distinction between Luni-Solar Precession, which Newcomb describes as the motion of the celestial pole, and Planetary Precession, which is the long-term change of the plane of the ecliptic relative to a fixed frame of reference. These both have an effect on the motion of the equinoxes and Newcomb defines the combined effect as General precession. Planetary precession also causes the obliquity of the ecliptic to change over time.
Planetary Precession is not really precession. Perhaps we could split the article into Precession (the general physical/mathematical phenomenon associated with a rotating body) and Precession of the Equinoxes (the astronomical phenomenon).
--Portnadler 19:45, 5 December 2005 (UTC)
thread_title:  General, Luni-Solar and Planetary Precession 

message: There's also a webcomic http://precession.eviscerate.net/ called precession... does this need disambiguation? The webcomic in question doesn't have a wp page yet. —The preceding unsigned comment was added by 217.39.163.153 (talk) 16:03, 6 December 2006 (UTC)
thread_title:  Disambiguation? 

message: Under precession of the equinoxes there is a statement that one degree of movement takes 180 years "from the point of view of the observer." This leads to a full cycle of 64,800 years.
Shouldn't this be corrected to the 71.6 years per degree referenced in the Precession of the equinoxes page?
--jb 15:52, 15 December 2006 (UTC)
replies: text: The figure is correct. The celestial poles move 1 degree per 180 years. Since their circle is only 23.5 degrees in radius, 1 degree of that circle (which is 1/360 of a full cycle) takes just 71 years. The way, the truth, and the light 01:32, 11 August 2007 (UTC)

text: They are the same angle - that between the Earth's axis and the ecliptic plane. The way, the truth, and the light 01:32, 11 August 2007 (UTC)
thread_title:  Error in precession of the equinoxes? 

message: I am surprised to learn that the precession involves a 23,5 degree angle, exactly the same as the 23,5 degree angle of the Tropics. Is there an explanation for this? — Xiutwel ♫☺♥♪ (talk) 07:31, 6 August 2007 (UTC)
replies: text: They are the same angle - that between the Earth's axis and the ecliptic plane. The way, the truth, and the light 01:32, 11 August 2007 (UTC)
thread_title: 23,5 degrees

message: The paragraph goes on to discuss loosening of bicycle pedals. I have no doubt that this is called precession, but I'm sure that this is a completely unrelated phenomenon. It is a purely mechanical process and does not depend on inertia and is not inversely proportional to spin rate. I suggest it get its own article or at least a separate section that explains the difference. Any other thoughts? -AndrewDressel 14:46, 25 December 2006 (UTC)
replies: text: Created separate section and explained the difference. -AndrewDressel 03:02, 27 December 2006 (UTC)
thread_title:  A disadvantage of precession ... 

message: This sentence did not make sense:
:Only moving objects can be in torque-free precession. 
After all, only moving objects can be in precession at all. I have attempted to revise the section. Mark Foskey 13:19, 30 August 2007 (UTC)
replies: text: I found a reference that allows one to get the (new) equation. Done? Dan Watts (talk) 13:45, 8 May 2014 (UTC)
thread_title:  Torque-free precession 

message: I'm sorry if this question has already been answered before in the article or the talk page, in that case I missed it.
I'm just curious about this, when we apply a torque to create the precession, we are doing real work, because we apply a force over distance (and of course torque is just angular work). Still I wonder, where exactly does that spent energy end up? Does it reduce the speed of the wheel somehow, has it gained some potential or is it just missing ;D ????
I'm not trying to act dumb, I just don't see where that energy goes....--Nabo0o (talk) 12:25, 18 November 2009 (UTC)
thread_title:  Where does the work go? 

message: I'm very sorry to admit I don't understand the cause of gyroscopic precession at all. Anyway, I wonder whether there can be precession (the gyroscopic or torque-induced type) in a rotating perfectly round ball (supposed it is possible to exert torque on it). I had thought precession of the Earth (or Mars) was an example, but Axial_precession says that this is due to gravitaonal forces and would not occur if the Earth were perfectly round. I'm not sure about Mars here. Can anyone show me a real-world example of a precessing, near-perfect ball? And what about precession in rotating black holes? Would that be produce a specific type of gravitational waves? Thank you very much. Illegal604 (talk) 11:11, 22 April 2010 (UTC)
thread_title:  Precession of a ball (sphere) 

message: I may be overlooking something, but it seems to me that the rotational axis of the gyroscope illustrated in the first picture in the article, is a constant vertical axis through the bottom pin of the gyroscope. Since precession is defined as "a change in the orientation of the rotational axis of a rotating body," I wouldn't think this is an example of precession. In any case, it isn't in general the case that one side of the precessing object is always faced with the z-axis, so this is a poor illustration of the general phenomenon.  — Preceding unsigned comment added by 94.224.49.243 (talk) 07:15, 2 January 2013 (UTC)
thread_title:  Precession of a Gyroscope illustration 

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message: Earth precession is told to be around 26000 years. I would like to know how exactly this has been determined. One argument involves accurate measurement. But as far as i know, it is impossible, no matter how accurate you measure, to determine this with any reasonable certainty if the measuring time is so short. This follows from Heisenberg. If you have less than one period measured, uncertainty about the exact period will increase. With less than 1/10th of a period, one can at best fit a 1st degree polynomial through the data, but never a sinuoid. You can create artificially a perfect sinusoid (e.g.with matlab/octave) from which you know the period, but I was never able to recover the exact period from the datapoints if not a complete period is in the signal. Not with harmonic decomposition (which wil at best lead to a probability distribution) and also not with more exotic datafitting methods such as fminbound, fninsearch, fmincon, fminunc. Please show me a good article that explains how earth precession is determined — Preceding unsigned comment added by 86.93.56.137 (talk) 18:40, 30 April 2014 (UTC)
replies: text: "it is impossible, no matter how accurate you measure, to determine this with any reasonable certainty if the measuring time is so short" - yes, and that's why the Earth's precession is said to be around 26,000 years. The three zeroes at the end suggests it's only accurate to within about 1000 years. 212.9.31.12 (talk) 12:34, 28 January 2016 (UTC)
thread_title:  heisenberg uncertainty principle 

message: My response to this article, "Is this supposed to be comprehensible?"
...at least the first quarter, after which I quit.
I'm thinking, of all the dark secrets of technology, maybe they want to still keep this one a secret by making it seem incomprehensible?
Or maybe our poor experts just aren't talented at technical writing for comprehension?
If I have to learn this subject, and I may because it's on the trailing edge of cutting edge technology across so many areas ...
I will rewrite the entire thing - without any non-essential waste of anyone's time.
Xgenei (talk) 01:57, 4 November 2015 (UTC)
thread_title:  Disappointingly Soupy technical writing. 

message: Okay, I know this isn't a physics forum, but something just doesn't make sense about the explanation given in the "Classical (Newtonian)" section.
:Actually, the force attempts to displace them some amount horizontally at that moment, but the ostensible component of that motion, attributed to the horizontal force, never occurs, as it would if the wheel was not spinning. 
How can the force "never occur"? If you push (instantaneously) on the rim of a wheel, spinning or not, that point must move, surely? Even if it only moves a little bit and immediately moves back. It can't offer infinite resistance to any force.
The force can't be instantly "teleported" to the 6 and 12 o'clock positions. That obviously doesn't happen when the wheel is stationary (the point pushed will definitely move in that case), and it doesn't noticeably happen if the wheel is spinning slowly. There isn't some cut-off speed where suddenly the force takes effect elsewhere.
Right? 212.9.31.12 (talk) 12:32, 28 January 2016 (UTC)
thread_title:  On the simplified explanation with the bicycle wheel 

message: Jeez, what is the point of this page if it cannot define precession without referring to Euler Angles?
This is yet another wiki page meant to serve wiki editors getting their phds then to serve any actual humans looking for an explanation.
I'd rate this page a D in terms of helpfulness, and yes, that actually is higher than the rating I give to most wiki pages. 107.3.134.101 (talk) 21:29, 10 September 2022 (UTC)
thread_title:  If this page can do no better than to define precession in the first paragraph in terms of Euler Angles....