Why don't astronomers consider precession as a main movement of Earth?

[Franco Malgari]

Astronomy considers only two main movements of Earth, rotation around its axis in a day and the revolution around the Sun in one year;
because astronomers do not consider the rotation of the Earth's axis around itself in a year?

 

[tiny-tim]

Welcome to PF!

Hi Franco! Welcome to PF!

Astronomy considers only two main movements of Earth, rotation around its axis in a day and the revolution around the Sun in one year;
because astronomers do not consider the rotation of the Earth's axis around itself in a year?

I'm not really following you …

the Earth's axis rotates once every 25,772 years, not once a year … see http://en.wikipedia.org/wiki/Precession_of_the_equinoxeshttp://en.wikipedia.org/wiki/Precession_of_the_equinoxes

(or do you mean the extra day … the Earth rotating 366 1/4 times in a year of 365 1/4 days?)

 

[Franco Malgari]

Te Earth's axis remains in the same position throughout the revolution around the Sun, turning always to the stars.
If I made a model of planet earth-sun I forced to use a motor to spin the globe in 365 days just to keep the Earth's axis always stationary relative to the revolution around the Sun ..
It's not precession..

 

[Integral]

What is the effect of this "third motion"? It does not seem to show up in the many long term photographs which are taken by the Earth's telescope.

Perhaps we are still not understanding you.

 

[Franco Malgari]

Please, read on Google "third movement of Copernicus":

 

[Franco Malgari]

This is the question:

 

[Borek]

If I understand the situation correctly there is no third movement, so there is no need to consider it. It was just an artifact of older models.

If you put a small globe on the turntable, axis will change its direction in the space, and it was already obvious in precopernican times that it is not happening - Earth axis always points in the same direction. To explain the difference between this type of the model and reality third movement was introduced. But Earth is not revolving around the Sun as if rigidly attached to some disc, it is more like a gyroscope.

 

[D H]

Please, read on Google "third movement of Copernicus":

Copernicus' third movement was a completely unnecessary and useless encumbrance. Copernicus envisioned that the natural motion of the Earth would be to have the Earth's axis always tilted toward the Sun. Since this natural motion wasn't what was observed, the axis must have been rotating so as to keep it aligned with the fixed stars.

A better model of the natural motion of the Earth's rotation is that the Earth's rotation axis would remain fixed with respect to inertial space unless torques act to change the Earth's angular momentum.

Astronomy considers only two main movements of Earth, rotation around its axis in a day and the revolution around the Sun in one year;
because astronomers do not consider the rotation of the Earth's axis around itself in a year?

That is good enough for most amateur astronomers, but not for professional ones. Neither the Earth's angular momentum nor its angular velocity are constant. Astronomers worry about precession, nutation, polar motion, changes in the Earth's rotation rate, etc. There are thousands of terms to the IAU 2006 rotation-nutation-precession model, and even that doesn't capture all of the motion. There are variations that can't be modeled (yet). If you need extreme precision in the Earth's orientation (e.g., microarcsecond pointing) you have but no choice to use the polar motion and ΔUT1 values which are updated daily by the IERS.

 

[tiny-tim]

Please, read on Google "third movement of Copernicus":

Franco, this is ridiculous …

all the references seem to be to forum threads such as this.

If you have an proper reference, please specify it in future instead of making us hunt for it.

There is no third movement of Copernicus (other than a historical mistake).

 

[D H]

Franco, this is ridiculous …

Wikipedia article on Copernican heliocentricism, http://en.wikipedia.org/wiki/Copernican_heliocentrism

Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.​

J.H.Bridges, Tycho Brahe, in Contemporary Review, February 1902
http://books.google.com/books?id=30IeAQAAIAAJ&pg=PA197

The third was a movement by which, in revolving around the Sun, the Earth maintained its axis in a direction parallel with itself during the whole circuit. Without this third movement Copernicus imagined that the Earth's axis would always be inclined toward the Sun (or rather toward the axis of the ecliptic) at the same angle ...​

Hilary Gatti, Giordano Bruno and Renaissance Science,
http://books.google.com/books?id=9cYumhwTQP8C&pg=PA73

Having defined his third movement in terms that allow the axis of the ecliptic to remain constant, Copernicus then explains that this constancy is not absolute. The direction of the axis slips back very slowly over the centuries with respect to the sphere of fixed stars, giving ruse to the phenomenon known as the precession of the equinoxes. ... By explaining the precession as a very slight movement of the axis of the Earth in its orbit around the sun, Copernicus had much simplified the phenomenon of precession, and he was quick to point this out as a distinct advantage of his system over the traditional geocentric astronomy.​

So essentially Copernicus' third motion is precession, but stated in a form that is a bit non-scientific, at least by our standards.

 

[Borek]

tiny-tim, I think the turntable analogy explains it pretty well. I can try to take a picture if you have problems figuring out what I mean.

 

[Franco Malgari]

But if I had to build a model, I use three motors:

 

[Borek]

First - model is not the reality, model is just a model. Sun/Earth system doesn't contain a single motor, yet it works for over 4 billion years.

Second - you don't need three motors. You can use M1 and M3 only, just mount the globe on the bearing and gyroscopic effect will take care of the axis stabilization, just like it does in reality.

More importantly, if you could build a friction free model without motors, and put it into motion by spinning the Earth and pushing it around the Sun, it would replicate the real Solar system for ever.

 

[russ_watters]

This is the other side of the coin from the more common problem people have with the two motions of the moon...