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Am I right that the difference is that precession indicates the direction of the inclination of Earth's axis and the tilt is the quantity of this inclination? So that when there is no tilt there can't be any precession?

And are the inclination degrees of the precession and the tilt related or the same?

It seems that the precession causes different possible polar star, but does the tilt also have an effect on this?

Perhaps the above is not right. If not, what is the difference? Is, perhaps, the precession just the changing of the tilt?

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  • $\begingroup$ Why post the same question here when you have already asked it on Astronomy SE? $\endgroup$ – Dr Chuck Jan 31 '17 at 9:49
  • $\begingroup$ Please don't multi-post. $\endgroup$ – gerrit Jan 31 '17 at 18:53
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enter image description here

The picture above of a spining top is a model for the Earth. Precession is causing it's axial tilt to rotate in a circle.

So that when there is no tilt there can't be any precession?

Yes, without an axial tilt, no precession would occur, and the Earth would revolve in a fixed position, relative to the faraway stars. We would always have the same star pattern above our heads, with the same Northern and Southern pole stars (if there was a star lined up just right on each hemisphere.).

And are the inclination degrees of the precession and the tilt related or the same.

The tilt is relatively constant, 23.5 degrees, but the precession, which in the case of Earth takes 26,000 years, means that the axial tit will point, in relation to farway stars, in different directions.

However, Nutation creates a small but significant in the angle of tilt of the Earth towards the Sun. This has the effect of altering the position of both the polar circles and the tropical circles.

The moon is also involved in this process, the greatest component of the nutation of Earth is similiar to that of the precession of the orbital nodes of the Moon, with a period of 18.6 years.

My thanks to Jim for pointing this out.

Our pole star is now Polaris, in the Ursa Major constellation, but in 3000 BCE, the faint star Thuban in the constellation Draco was the North Star.

Tilt And Precession

The phenomenon we call "precession" was discovered by Greek astronomer Hipparchus when he compared his own circa 200 BC records with older charts. What he saw was that the equinoxes in his day (where the sun's path crosses the celestial equator) were in a different position among the stars than the 150-year-old comparison charts showed. This is due to a gyroscopic wobble of earth's spin axis that takes 26000 years to complete. In this wobble motion, the tilt of the earth stays roughly constant at 23.4 degrees but the orientation is always changing.

Changes in Precession Rate

When, in about 1,500 million years, the distance of the Moon, which is continuously increasing from tidal effects, has increased from the current 60.3 to approximately 66.5 Earth radii, resonances from planetary effects will push precession to 49,000 years at first, and then, when the Moon reaches 68 Earth radii in about 2,000 million years, to 69,000 years. This will be associated with wild swings in the obliquity of the ecliptic as well. Ward, however, used the abnormally large modern value for tidal dissipation. Using the 620-million year average provided by tidal rhythmites of about half the modern value, these resonances will not be reached until about 3,000 and 4,000 million years, respectively. However, due to the gradually increasing luminosity of the Sun, the oceans of the Earth will have vaporized long before that time (about 2,100 million years from now).

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  • $\begingroup$ What do you mean that the tilt is always the same (23.5). I read that it can variate between 22 and 24.5 degrees. I also read that the precession variates between 21.5 and 24.5 degrees. Although I can't even imagine how the precession can have an angle on its own (independent of the tilt) it looks like that there is a slight difference of the degrees between the tilt and precession, how to understand this? $\endgroup$ – Marijn Jan 31 '17 at 9:01
  • $\begingroup$ I edited my answer, providing a source that says the axial tilt is constant at 23.4 degree, although the precession is always changing the direction of this tilt. That's seems seems ok to me. Over a very, very long time, the pull of the moon will reduce and the pull from the other planets will change the rate of precession, but these are extremely long term effects. I don't know either how the precession can have a tilt of it's own, but maybe put the source of this claim in your answer. $\endgroup$ – user140606 Jan 31 '17 at 9:55
  • $\begingroup$ @Jim Thanks very much, I can see now why my source said the tilt was "roughly" constant. I have edited the post to reflect this. $\endgroup$ – user140606 Jan 31 '17 at 14:12
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Precession is the phenomenon that the axis of rotation changes with time.

Tilt describes the angle of the axis of rotation at a given moment in time relative to some reference.

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  • $\begingroup$ And are thing I wrote right or wrong? $\endgroup$ – Marijn Jan 30 '17 at 18:57
  • $\begingroup$ The things you wrote are somewhat confused and inaccurate. "Precession" is the CHANGING of the axis of rotation, you stated "precession indicates the direction of the inclination.." which is not correct. $\endgroup$ – Floris Jan 30 '17 at 18:58
  • $\begingroup$ Can I also say changing of the direction of the axis? If so how to understand the inclination degree? $\endgroup$ – Marijn Jan 30 '17 at 19:10

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